GG,
GMC did use Buick sixes, around 1929-1932 or so.
They also used a Pontiac flat-head six in the light & medium duty trucks of the mid-'30s.
In the '50s, they again used a Pontiac engine -this time a V-8.
I think the '37-'54 sixes were GMC designed and were their own animal; they had insert rod bearings, whereas Buicks kept their beloved poured babbit rods until then end of the straight-8: 1954!
In the late '20s, Buick had the biggest and best valve-in-head sixes of all the GM divisions, so that was probably why they turned there for a new "truck engine".
Will try to come up with more details and get back to you.
Frank McMullen
