[franken]

My thought is the 420 and 465 are comparable, and great for their day, and designed to the driveline.
A truck w/ x rating, came with an engine and therefore driveline based on options decided by the buyer, or bought off the lot.

Or rather start w/ the engine. A small six would have a manual trans and high numerical rear gear.

The point is, match the engine, trans, and rear gear.

NV4500 likes a late NP205, or adaptor. Look around. you can get early adaptors but look for a round pattern NP205, as a start.