Don't go crazy, a 250 should do everything you could ask of a truck. The factory rear axle is good up to about 300 hp.
If you need more than a 250 can give, and still crave a 6, build a 292. Of course a small block will also bolt in.
The 302 is an awesome motor. A very nicely built street motor will cost more than $5000 and be limited to less than 250 hp with about 300 lbs torque. As I'm sure you are aware... 302 gmc motors don't fit well in chevy trucks because they are longer than the chevy motors.
I have a 59 gmc 1/2 ton longbed with V8. The original 336 v8 was rated for about 205 hp. The truck would do 60 mph all day long, and haul anything. I swapped to an OD transmission, and the truck would do 70 mph all day long and still haul anything. The 336 was tired, so I swapped it for a 59 pontiac 389 that was bored and stroked for 447 inches. The pontiac V8 was a bolt in replacement for the 336 gmc v8. The engine was too much for the factory axle, and ripped the housing apart within 30 miles of the swap.
If you really want to work your truck as a truck, lose the 3 speed. A 4 or 5 speed transmission can really help.
