more important than the source is the build quality. You can get a stock stall or low stall TH350/400 TC with high quality stuff online and sometimes through a local parts place... they'll possibly charge you shipping if its a special order.
GM might even still sell some for their TH400m TH375s that they put in motorhomes behind 454 big blocks. Shop around, man. But make sure you don't go cheap. You want a quality Sprag and "furnace brazed" yadda yadda.
I've heard a Allison TC will work too, (behind a Duramax turbo diesel), but never tried it.