No need for an ifs clip. The big issue is the steering box. The clearance is very tight for the exhaust manifolds. I've seen some guys simply moove the box to the left about an 1 1/2"-2" to get clearance, or the engine could be mounted off center, too. I used to have part numbers for Hooker headers that were supposed to clear, I think one side was a Chevelle fitment, the other was '67-'69 Camaro/'68-'75 Nova fitment. I don't recall which side was wich, though.
The NV4500 would be a good trans choice, but a 700R4 would also be ok. GM used them in their light 3/4 tons for years. My Dad has an '88 with one, we towed my truck back from Marrietta GA with it, 3700# plus trailer weight, did just fine.
You mention a 4x4 trans. Is a 4x4 conversion in the works as well? If so, you might look for a '73-'87 3/4 ton or '79-'91 1 ton for axles. Should be able to get better gearing, plus front disks. You may need to contact Advanced Adapters for a T-case adapter, depending on what axles and transfer case you use. I would suggest the NP205 gear drive part time case.
