That GMC 305 V6 was an absolute pig.
I'm not sure by what standards ... I hauled GCVW of something way north of 20K pounds. Obviously you don't go very fast in the mountains with that kind of load and that small of a gas engine. Most of the roads were so damn rough that you couldn't go very fast anyway.
It had a lot of torque and got pretty darn good gas mileage considering the loads: about 10-11 mpg loaded or empty.
I never drove one in a pickup.
They did have a nasty habit of tossing rods through the side of the block, so I never pushed the rpm too high out on the highway. I bought a complete "windowed" engine for something like $50
just for all the misc parts attached.