As Andy said, it would depend on whether it is a 1/2 ton or 3/4 - 1 ton. I put a turbocharged low-mileage 6.2L CUCV takeout diesel in my '67 C-30, but since it is a 1 ton, I upgraded to a 14 bolt, which is plenty adequate. I have a slighty turned up DB2 4911 injection pump (which is the hottest mechanical pump available for 6.2/6.5's), I added a stud girdle to strengthen the bottom end and ARP head bolts, and I plan to run 10 - 12 psi boost. That is about the max amount you can run on these engines with without lowering compression ratio. They will produce between around 215 - 225 HP and around 440 - 450 TQ tuned this way. In stock form without the turbo and 4911 IP, the CUCV J code engines are rated at 155HP and 250 TQ. I considered a Cummins 4-BT swap but they are very noisy and vibrate badly. I also thought about a 6-AT since they shorter than a 6-BT. I even considered 3-53, 4-53 and 6V-53 Detroits, but in the end, the 6.2L swap was the easiest since it was a direct bolt in, and had the correct pattern for my transmission. The Detroits are still the sweetest sounding diesels ever built though, IMO!
I'm running a NP-435 4 speed as of now the 14 bolt has 4.10's in it. I'm going to swap them to either 3.42's or maybe even 3.21's to make it better out on the road. I had a 6.2L in a '84 K-10 with 3.08's and it towed out on the road better than a '83 C-20 with 4.10's that I had at the same time. Both were automatics, and the 4.10 geared truck took off a little better, but it struggled on hills out on the interstate worse than the 3.08 geared truck. The 4.10's put it so far above its torque peak at 70-75 mph that it towed better, slower in high gear. That being said, I'm not afraid to go that high with my '67 C-30 since the NP-435 will have no issues taking off.