I used an '88 GTA t-5 transmission in my '69. Installation is fairly simple. The console, if you have one, needs to be shifted back about 2 inches. The drive shaft has to be shortened a couple of inches as well. There are some other minor mods as well.
It is holding up just fine behind my 350.
T-5s can be found usually in the $400-$650 range. I was lucky to find mine for $300 plus $80 for shipping. The only other major expense was the driveshaft work with new u-joints for $120.
If tire smoking is high on your list it won't hold up. Other wise it is an economical way to get over-drive.