3 1/2" exhaust?


I hope you just mean 2 1/2". If you have 3 1/2" I'd spend your money on a new exhaust system instead of an EFI kit. 3 1/2" would KILL your power.
There is no way to "convert" from C4 to C6, or from 3 spd auto to 4 spd auto. I would definitely keep your C4. If it needs a rebuild, get one, put a shift kit in it and a decent stall converter. C6s are bulkier/heavier and not necessary when you have a good C4.
I do agree with these guys though, I wouldn't bother switching to EFI. If you're doing it for a "custom" thing then I suppose it's your choice, of course, but if you're looking for performance out of it, it's not worth it. Just buy a higher cfm carb for $200 and be done, instead of a whole lot more for an EFI kit.