I've used Molnar rods and cranks on several builds o
I'm a big torque seeker, especially on street and strip motors. On your deal I would go with the 400 block and either 4.25 or 4.300 stroke and a 6.700 long rod with a custom piston built to the specs you want for compression ratio, fuel type and valve to piston clearances
The 3.91 stroke is a good choice also, just not as much C.I. and torque compared to the longer strokes