[POWERSTROKE]

Welcome to 60-66 GMC. I can positively assure you both rear wheels get power. It's the nature of an open rear end to send power to the rear wheel with the LEAST traction. There's an entire industry built up making locking or torque biasing differentials. Unfortunately nothing available for an axle as big as your #4000 GMC has.

With the 305 V-6 and a 4+2 trans & axle performance loaded will be lethargic. I spent a L-O-T of time decades ago rowing through 5+2's with substantially larger engines but substantially heavier loads.

But, your truck looks in EXCELLENT shape for being 53 years old. Somebody slammed the drivers door open, shame on them! Dad had 1960 Chevy 3/4 ton pickup with similar sheet metal I may or may not have dented similar to yours.

Does your truck have a PTO operated hydraulic hoist for the box? Many did.