Everything Four on the Floor said.From my experience,all replacement stock style 261 oversize pistons place the piston around .040 below the deck at TDC.The original GM std size pistons are .020 below the deck.Fitting whatever flat top pistons close to deck height,lets say .010 below the deck by milling the block,using a 848 head with a clean up milling job,.010-.020,will yield about 9-1 actual compression,about all the compression ya can run on available pump gas with this head design.
Back to the original question,the GMC has a better breathing stock head than any 235/261,the 302 type head being the best.The head difference is worth maybe 20 percent more power on a modified engine,all else being equal.Another advantage of a GMC is larger displacement,up to 302 cubes stock,on the street,bigger is better.
