Absolutely right.a 400 with overdrive would be the swizzle.
I have a nv4500 in the garage. If the 6.2 lives, I'll turbo it and put the 5 speed on it.
My k5 has a th350, but I scored a nv833 for it.
It's a a 4 speed, with 4th being overdrive. It had a short run in the 80s, if you find one grab it.