[fatbass]

I'm late to the party but I love this truck. I had a '71 K10 shortie the same color scheme and always wanted a 3/4 ton to make a pair.

Is there a chance someone did something to increase cylinder pressure/ compression ratio? Wrong head gasket, pistons, poorly machined heads, etc...
In the old days of points ignition (poorly maintained) and poor gas/air filtration, carbon buildup on pistons and head surfaces was a problem to the point of causing preignition.
I would do a baseline compression check. If wildly high or erratic readings are seen, scope your cylinders to check for carbon buildup.