well im almost done with the paint job on the 84, and after i get done with that reassembley will be on the way, and i dont wanna be tempted to hook my driveshaft back up to the old 10 bolt 2:73 thats in it now. I have a 2800 stall convertor, and my engine is susposed to make the most power from 2200-7800 rpm (its balanced, so maybe it wont blow up) what would the best rear gear be to get in the mid 12s? Oh and keep in mind economy went out the window when i built the engine, it could never get over 1000 rpms and still only get 9 mpg

 

3.73's will get you about 3500 rpms on the freeway at cruising speed.

3.42's will be around 3100. Its your pick.

 

If you want this rear end to have a chance at living then I'd recommend a new 28 spline Eaton posi and a set of 28 spline axles. A T/A Performance cover would also be a good idea. The 26 spline parts that your '84 will have are probably not going to hold up to 12 second ETs. The whole rear end might not hold up to it, but it will have a better chance with these parts. As always, correct ring & pinion installation is very important to the strength of the rear end. Pinion depth, backlash and bearing preload are very important.

 

I run 3.36 with my th350. small block supercharged. goes 11's at 116 and comfortable.
I put in 4.11's and dropped to 108mph and hated the freeway so changed back. et's were similar 60 foots went from 1.7 to 1.6 and eighth mile was faster but hurt top end.