Yes, you can buy them practically anywhere. Check them out on
Summit http://www.summitracing.com/search/Depa ... Rank%7cAsc
Jegs http://www.jegs.com/i/Metco+Motorsports ... 0/10002/-1
... but first, you need to know the transmission.
... I believe the generic ones are all made according to the length of the driveshaft behind a 200-4r
for a 700r4, like the one I'm using, you'll need a 3" set back loop (that's 3" farther back then where the 200-4r loop is).
I believe the generic one also works for the th350. But for a th400, you'll need to figure out the correct length.
the fellow at http://fastmonte.com/
sells ones he's made with the 3" setback.
there are also "universal kits" where you can bolt/weld together your own one for a custom fit.
83 GP, 350, 700r4, 8.5 10bolt, 3.42 gears, 16" IROC wheels, 13.6sec @ 103mph (on tires), 12.6sec @ 105mph (on slicks)