I don't see anything about a gear change. Did I miss that part of your build? Won't really matter what trans. you have if you don't change the diff gears it will still be a slug so to speak. Change your gears, 4.88's if not 5.29's.
The A340 trans is not an unreliable trans at all or weak, it's found behind Turbo Supra's and V8 Lexus' they are kinda fast cars.
Since I've never done a 2wd to 4wd swap I would be vary of spending more money on a 2wd trans that I can't use for my 4wd conversion.
With an auto ECM you will throw a code if the trans is no longer there, so you'll fail inspections. Marlin touches on this in the 3RZ swap thread.
Buy a wrecked complete donor truck and then you'll have everything you need, that's pretty much how I approached my swap.
