i used to have an open diff with 2.44 gears with a 255/60/R15 tire and rim combo behind my 360 with the worlds worst intake ever (torker 340) LOL and even that would spin the tires on take off.
No one has mentioned timing yet. Sounds to me like there is something not working right in your combo, especially with your 4 speed that car shouldn't feel like a slug unless the timing is out or the cam is not installed (degreed) properly.
Go through the timing numbers, doesn't cost anything to check and a retarded timing will make any performance car feel like your grandmas groceries getter LOL.
Hope you'll find your culprit.
