If you've built the engine, chances are you can do the trans. There are some tools you might need, but you can probably rent them from somewhere.
You say you're looking for a performance unit. Here's where it could get sticky. There are some really good performance parts out there like hardened input shafts, steel front drums, aftermarket planetaries, hardened sun shell and sun gear, but they don't come cheap. Reliable performance costs money. However, if you don't have huge horsepower, I might suggest:
Master overhaul kit with Alto friction plates. You can also use the high energy frictions from the A518 in both locations, might be a little cheaper than the Alto kit. Replace the steel plates in all cases. The high engergy front band as used in the A518 transmission (it's a flex band, but a good one), a rear band, a bushing kit (if nothing else, make sure you change the pump bushing), a washer kit, the spring and roller kit for the rear sprag, a brass filter, an intermediate servo piston (Superior makes one), heavy duty band strut, reverse servo piston (Sonnax), and a quality shift kit (Transgo TF-2 is the best one). There's a bearing in the extension housing, check it out closely. Look at the pump gears and replace if they look worn.
Take your time, look at the video, clean everything spotless and you should be good to go.
If you're thinking about the hard core performance stuff, check out this company:
www.tcsproducts.com . They make all their own stuff and supply it to others.
