Bought an 81 gmc new, had a 292 I6, Muncie 4 sp and 4.10s. 8600 gvwr, 8500 gcwr. IOff the line, loaded on steep grades IE over 15%, it would out do equally loaded 88 tbi 454, 89 4 bank 454 3500s I had with th400 tyrannies. Blew those trannies up severely 30k miles like clockwork! Stalled them out a lot also. 292 combo I.never stalled it out! Both 3500s would have been better with 3.42 or 3.73 gears and the Muncie 4sp.
All in how the trans, axle gears are in relation to HP and torque to rear wheels. Some setups are better than others.
Today's rigs are better setup generally speaking. I'd put OPs motor in my 26k Navistar, gm 6.0 too! They would do fine. Especially when at one time, you could get a GM/Ford 292/300 I6 in a 26k gvwr 65 series truck!
Marty