A 6 will nearly alwyas get better gas, and a 8 will have more power. Not sure if your wanting speed power, or hauling power. I do believe up to the later 70 early 80's that most nearly any trans/ang will bolt right up, same w/rears. Of course you may need a adap kit for some. A 6 trans is better, if your running a higher geared read. If a lower geared rear, use a 4 possibly a 5 depending on the ratio's.
Not much help, but it is some info. Also, you've beat me. I am 26 and I believe there is a fellow here that is 24. 18 huh, you a punk?
We dont like punks here
Best of luck, and have fun!
