The TH400/NP208 is a stout combo. Admittedly it's no 205, but GM put it in the K30 trucks with autos including the CUCV Military Trucks. Unless you are planning a monster big block and 44" tires you will be fine.
I have bought a couple of square body Subs as donors for my Jimmy and my kids Jimmy as well. First was an'86 with all sorts of HD options and it was a TH400/NP208 truck with optional 4.10 gears in the 10 bolt/semi-float 14.
The second an '84 with 3.73 gears in the same axles. It is a TH700/NP208 truck so you don't know what you will get. But like you, this seemed a cheap way to get reasonably Heavy Duty parts. I paid $300 for the '86 and it had $200 of Magnaflow mufflers, headers and a Performer intake! I told my buddy about this deal and he said "I'd have sold you our '84 for $200." and I said you just did!
