Ended up trading a short-block that I had for a rebuilt 1963 389 .30-over engine a friend purchased in Illinois. The rebuilding of the 421 that came with the GP will have to wait for resources. The replacement has 10/10.5:1 compression, HO rods, Mellng oil pump, mild cam, Edelbrock Performer intake, 1965 "76" heads, and timing chain cover and water pump are 1969-ish. Told it's about 410-420 HP, and about 450-475 lbs. torque. Will require a higher octane gasoline. Was built by a reputable builder who I'm trying to get into contact with to learn a little more. Should be plenty of power through the 3.64 posi rear-end.
With all the junk on it:
Test fit the long-branch manifolds, alternator, power steering pump, oil filter adaptor (to clear the manifolds), engine mounts, carburator, and AC bracket. Of course, there are 'opportunities' associated with the non-stock setup. The passenger side engine mount interferes a hair with the manifold. Because the 11 bolt water pump extends further forward, there is a pulley alignment issue which will need to be resolved. It's unclear whether the NOS 421 +/- 500 CFM AFB Carb that I have will be "enough" for this powerplant.