The first tandem truck i drove was a ‘71 GMC gas job with a 10yd dump box on a gravel haul. 427, 5&4, hydraulic brakes. Lots of hills on gravel roads. Just about wore my right arm off rowing that 5&4. Some split shifts with left arm through the steering wheel so i could row both sticks at once. I could never catch the other identical truck until the day he ran out of gas. He burned about 20imp gallons more on a long day load for load. I found out then he had an allison automatic. Probably an MT650. I once drove a GMC6500 gas job with a 454 and 13spd. More torque than a 427. Apparently they were only made for a couple years ‘78 or ‘79? Trans was maybe a RT or RTO 6613?