I've found a piston/rod/stroke combination that looks like it will work on most of the GMC engines with up to a 1/4" increased stroke. It involves using a custom length H beam rod for a big block Chevy V8, one that's rated for over 1000 HP on drag race engines. Those rods are surprisingly affordable from some of the vendors on Ebay and the BBC rod bearings will allow for a pretty good increase in stroke by offset grinding the crankshaft. Pistons for Japanese industrial Diesel engines are included in the mix. The reciprocating assembly also ends up considerably lighter weight. The 256 engine was mostly used in Canadian military vehicles. I've rebuilt a grand total of one of them, and had to modify the pistons to accept different thickness rings.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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