i have about $250 to spend so i have to choose one. i would be interested in your pumpkins but i dont think i have enough to get one. i plan to get a used rear end and rebuild it and swap in 3.73s.
I would say the shifkit.
You will still have some money left over (probably about 1/2), to go to whatever (or save). Plus the cooler will help your tranny (just flushing it will).
Having the firmer/hard shifts makes the car much more fun to drive IMO.
Ppl say gears will help a lot, and I do believe they should, but I didn't really notice anything as far as 1/4 mile with 4.10's. I may just need some more power to get good use out of them though.
If you found a good used rear-end right now, go for that, but if you can find one most of the time, then I say wait and do the shifkit/cooler.