Hi,
In the fortys and fiftys, the GMC had a larger engine which also had real oil pressure to the rod bearings rather than the dip and splash the Chevy had.
We swapped the gmc engine into the chevys for more scoot power.
The gmc had a much truckier looking instrument panel.
Starting in 1955 the chevy went to their 265 V8 and GMC used the pontiac 316 V8 painted red.
The GMC large trucks used the Olds V8.
Both the pontiac and olds had more torque than the small block chevy.
I don't know when they started using the Chevy V8 in GMC trucks which took away their individuality.
Bob B