You need to change the tail shaft housing so that the trans will bolt to the crossmember and so that the shifter is in the same place. You can also change the front section, but I don't think it's required.

which year would be a good strong 12A trans? what about the GSL SE trans?

I'd pick the one with the best gearing, but if there's a difference in strength, the later one's are probably slightly preferable.

gslse transmission has no real differences as far as strength is concerned.

Just a different fifth gear. (these are for 84-85)

12a 5th: 0.758

13b 5th: 0.807

i want a 5th gear that my car doesn't fall on it's face after i run 4th gear out.

Isn't that the point of an OVERDRIVE?

Buy a J-Spec S5 TII trans - .84 overdrive. Personally I'd rather have something in the low .7's for my car, get a little better highway mpg's

i don't care too much for OD.the problem i have and any other n/a car, after i shift at 120 from 4th it barely accelerates. i want it to keep accelerating if i want it to.

make more hp or change the final drive ratio

true. i need to replace my trany anyway. power is on my to do list.

well,i picked up another stock s4 trans yesterday. the price was right $50,so we'll see how it works. if it doesn't,i'll get a 12a trans.