Since the GMC 270 engines are getting scarce, and rebuild parts are either expensive or unobtanium, here's a way to get 270+ cubic inches from a Chevy 261, with the same stroke (and torque) of the GMC engine. The secret is in an oddball high performance connecting rod made for big block Chevy engines. It's an H-beam rod that's 7.1 inches long, just a bit longer than the GMC 7 inch rod. By offset-grinding the 235/261 rod journal a few thousandths the 3 15/16" stroke can be changed to 4 inches, and the rod journal can be ground down to 2.200 "- - - -the same as a big block Chevy rod bearing. By using a .060" oversize Chevy 265 V8 piston, the resulting bore/stroke/connecting rod setup produces 274 cubic inches, and the piston crown is .005" proud of the block. If you want zero deck height, just take a skim cut off the top of the piston with a lathe. A Ford 292 V8 piston, also 60 over, puts the piston crown .015" below deck height. Since the big block rod is thinner than the stovebolt rod, use shims at the top of the rod over the wrist pin to keep the rod centered in the piston.
The same stroke and connecting rod setup applied to a 235, using standard bore pistons for a 1955-64 Ford 223 six gives 248 cubic inches and puts the piston about .018" below the top of the block. Either engine, with the 848 cylinder head, will get a noticeable boost in compression from the increased displacement.
The advantage to the above modifications is that modern, more available "over the counter" parts can be used, with a minimum of fancy machining or buying spendy custom made parts!
Jerry
