I copied this from the old site, it always comes in handy when I run a Y-Framer. Original poster was dm440c.
65 – 73 Chrysler, Dodge, and Plymouth full size
a.k.a “Y frame” cars
The full size C body cars from these years all shared the same basic chassis and subframe with some subtle differences. The 65 – 68 styling had a squared, angular look and was known as the “slab sided” era for obvious reasons. The 69 – 73 was known as the “fuselage” era as these were much more rounded. The relative toughness of the two eras is very comparable and it is debatable whether one is better than the other. One thing is for certain, that is that the front bumpers of any of these years are not of the quality found in the later 70’s, although the 73 bumpers are the best of this era and the 73 frames have an additional gusset up front to add a little strength. Subframes are easily interchangeable, although the earlier ones bolted directly to the body and the later ones had rubber isolators. Small and big block drivetrains are a fairly simple swap as well in most cases.
Frames:
The one glaring weakness of the Y frame cars is the Y itself, which is the large open space at the very front of the main frame rails near the core support. The best fix is to weld ¼” plate top and bottom to “box in” this section. If this is prohibited by the building rules, the next best thing is to add two large bolts directly through the Y on both sides in such a way that the two branches of the Y are joined together by the bolts. If neither of these will make it through inspection, these are still good cars but the strength of the front end is considerably diminished and this should be reflected in the driving tactics.
The older cars bolt the body directly to the subframe, but the newer ones have rubber biscuit isolators in between. Remove these and bolt direct for strength. Also, there are six mounting points stock (not including core support) but there are provisions for two more, one on each side just in front of the rear mounts under the front seat. Drill holes if necessary and add the extra bolts here.
If allowed, subframe connectors will improve the stiffness of the car by connecting the front subframe to the rear frame at the spring mounts.
Changing the front bumper to a shock bumper, such as a 74 – 78 Mopar C body, 74 –76 Chevrolet, or some models of Cordoba or Volare, will be a big improvement over stock bumpers but this may come at a price. If the stock Y frame is not plated or bolted, it may be overpowered by these stout bumpers and bend in objectionable ways. If the rules require stock frame configuration, the best choice may be the 73 bumper.
Chassis:
Adjust the front torsion bars to ¾ of maximum preload setting by turning in the adjuster bolts. Using the full adjustment may place too much stress on the suspension components.
Use drum brakes in the front (spindles are interchangeable on all years) or if using front discs, block the front brake lines at the master cylinder and remove the calipers and caliper brackets to prevent wheel lock up.
Remove the front sway bar entirely. Cut off the two metal tabs from the frame where the inner sway bar mounts attach to avoid them bending into things such as the fuel pump.
Run chain through the floor and over the tops of the frame humps in the rear, then down around the axle- this will prevent the body from “humping up” away from the axle. This is especially important with wagons. Tighten to the point that the rear springs are close to flat. Many derbiers feel it is preferable on Mopars for the front to be higher than the rear. Never run without the rear shock absorbers, especially if no chain is used around the axle.
Clamp or use duct tape on the rear springs at any point where the end of an individual leaf meets the next consecutive leaf. The most important is where the second leaf meets the main leaf in the rear. If possible, relocate the leaf springs inward so that they mount directly in line with the main frame rails.
Reinforce the steering rag joint with chain, tape, or replace it entirely by cutting out a section of sidewall from an old tire and bolting it in. Alternately, a small u joint can be grafted in place of the rag joint.
The biscuit style motor mounts are prone to separating so the engines should be tied to the frame with chain, metal straps, or any other acceptable method.
If the lower radiator hose is on the driver’s side for a big block car, it tends to get into the power steering pump as things bend up and in. Switch to a later style (73 – 78) water pump and corresponding radiator that has the lower hose on the passenger’s side.
Body:
The trunks are tough but they are also very long. If left stock, they tend to bend where the trunk lid meets the rear roof pillars, then the entire trunk section goes upward very quickly until it is flattened against the rear window and pointed skyward. This is neither easy to see through nor is it an effective way to hand out damage to opponents, so steps should be taken to prevent it. Rules for individual derbies dictate what can be done, but the most extreme would be to tuck the trunk lid, notch the frame in front of the rear spring mounts, and tie it all together with Allthread from the trunk lid through the frame.
Alternately, different combinations could be attempted based on rules, but at a minimum the trunk lid should be hammered downward to create a concave surface that is more likely to compress and fold inward rather than upward. The exact placement and pattern is a matter of personal preference.
Older cars have the inner front fenders bolted directly to the subframe, but on the newer ones the inner fenders float with just rubber splash guards connecting them to the subframe. After removing the frame rubbers, it will add a lot of strength to find a way within your rules to tie the inner fenders to the subframe.
Hammer the sheet metal on the sides of the rear quarters inward to the trunk to ensure it folds together instead of blowing outward. This is especially important on the Fuselage cars.
Hammer the very bottoms of the rear quarters (the trunk extensions) up until they meet the trunk floor, then bolt or weld them together.
Cut the sheet metal around the rear tires and fold it up and out away from the rear tires, then bolt or weld it back together. If some clearance is not created, the metal WILL bend in and rub on the tires (or worse).
Cut away the sheet metal of the fenders in front of the front tires, also trim the ends of the bumpers, otherwise these will be rubbing on your front tires after a few good hits.
Weld or bolt together wherever two sheet panels meet or share a surface as rules allow. Of particular importance are every mating surface of the radiator support and also the inner front fenders. The rear frame rails are merely spot welded to the trunk floor, so this should be improved as allowed.
Fold hood and ends of front fenders around the radiator support.
The radiator mounts are not very strong, so tie the radiator to the support with additional pipe strap, wire, or ratchet straps
