You will have your work cut out for you trying to get lighter than a chevy. My 360 mopar motor was heavier than my 350 chevy. I don't see any way around that and still be legal. I don't think a 904 would be lighter than a powerglide but I havent weighed a 904 with a converter to compare. Have you weighed your 904 wth a converter yet? You will have to try to make up for that in the chassis while still maintaining enough rear percentage.