All Factory Best Bang??

Off the top of my head (I don't claim to be a B/RB expert, and I don't play one on TV):
Early 440 block ('66-'67 IMS) because of reinforcement ribs on the outside of the crankcase, possibly heavier casting all the way around.
"Six Pack" (HP1?) crank/rods/balancer (Heavier, I know; but you're looking to build torque with this motor, not spin the RPMs into the stratosphere).
902 heads, porting up to you. Or any of the "Stage" heads from DC/MP if they consider those "factory".
Early (late 50s-early 60s) factory solid tappet adjustable rocker gear.
Hemi pan (or not, since it's going into a truck) and 1/2" pickup tube.
Mid '70s iron TQ intake with a big old TQ on it.