Weak point is going to be the tranny. I do not like 200r4's and for good reason. 700R4's were bad enough.
I know you can put some good parts in them but even then it was meant for light duty work only and not in the back of a BB Olds.
That Olds puts out a ton of low end torque and that is what is hard on tranny's. I've seen those engines tear up built T400's let alone 200's.
Sounds like a great truck and well thought out. 455 Olds, good choice of engines for towing; can't be beat. Even better than the BBC IMHO.
Like others have said; post up some pictures when you get a chance.