I'm building a similar stroker engine, but I've figured out a way to do it with regular 261 pistons- - - -not the scarce and pricey Cadillac ones. There's also a NASCAR-style H beam rod for a Nissan engine that can be adapted to take the stroke out to 4 5/16 ", also with stock 261 pistons. That one requires some whittling on the crankshaft counterweights and piston skirts to keep those parts from getting into contact with each other. With a .040 overbore that one comes out to 292 cubic inches.
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
Love your enemies and drive 'em nuts!