Most of the head gasket problems are at #7 and occurred well before 50,000 miles. Yours may have been repaired at some time. GM's Bulletin tied the problem to misaligned intake gaskets which led to galvanic corrosion, which eventually eroded the head gasket (and plugged up everything else). Their solution was to counterbore the intake mounting holes for gasket retainers and current production, replacement heads, have this feature. Checking #7 is as simple as pulling the plug. Look for rust on the plug threads.