On a car with different heads, cams and pistons, one would logically assume that if they are on the same spec line (ala the 2nd gen RX-7), the weight would be set for the version with the highest potential.
In the RX-7 example, the 86-88 cars came with a 146hp 13B. Since the 89-91 cars are on the smae spec line, you can UD to that powerplant. The spec weight is based on the 160hp motor and it's potential.
That kinda makes sense. as you are making turning an 86-88 into a 89-91 car with the UD. However as Jeff pointed out wouldn't running an early intake on a later motor in either configuation be illegal?
In following wiht the RX7 model I am fine with changing the 86-88 model into an 89-91 model. What I don't see as legal is the mustang example. Meaning running a late model engien with an early model carbed intake, with a late model 5 speed, and an early model differential, in a late model chassis. The result is a car that never existed in any year from production.
If I could figure out legaly how to make this car turn and brake you could really take advantage of this rulling. In talking with Ron, I know they have spend hours and hours of R&D to make there mustang work. It does very well on tracks in the area.