It's too bad you didn't find an '81-'84 SM465/NP208 donor truck. This would have been much simpler.
that would have been too perfect haha
I have an '85 SM465 sitting in my barn right now. 6-bolt round-pattern T-case output, standardized length adapter (you can reuse your original, good driveshafts and T-case).
I'm keeping the hydraulic bell housing, cross member, and T-case, but I have no use for the tranny or adapter. And it just might make your life a bunch easier if I'm not too late.
P.S. - the Adapter does not appear to be cracked, either.
I hope so, the rpms just seem higher than they should be
Still got the original axel in 3.08
Running stock 31" tires? That should be a decent combination. It's a little high for a street cruising machine, but not too bad. My K10 runs 3.08s with 29" tires and a 0.7 overdrive, but that is the economy cruising package, not the tow rig package.
Did you have the truck running with the original TH400? That would have been spinning faster yet due to the non-locking torque converter.