Nice post, Skeet

Supposedly GMC 6 cylinder engines last longer than Chevrolet 6 engines, because the GMCs are harder due to a higher nickel content?

Yes the GMC grills were "bulkier", and the 54/55 Z-series grill was a monster/monstrosity. I admire that ugly grill on my Z-series GMC COE.

The GMC carburetors were/are far superior to Chevrolet Rochesters

Regarding lubrication, yes, GMC engines had high-pressure engines before Chevrolet changed in 1954. However, the 1958 Chevrolet 261 had a full-flow oil filtration system before GMCs.