The stock crap in those engines is bloody bombproof. I know guys here that used to buy 'em up and gas the hell out ov them, 300HP shot. It was never the pistons or bottom ends that quit. STOCK pistons mind you... and they made this distinction... jobber or rebuild pistons never lasted apparently. I've talked to a couple different guys that did exactly what you're doing... dumped a big ol' roots on top ov a stock low-comp 400 or 440, and they had probably more fun than anyone else i've ever known with a Mopar. You're good to go. I think you could easily get away with more boost and RPM...

Think is was Chrisf (the guy with the blown 383 Challenger) here that dumped a 6-71 into a bone stock mid-70's NewYorker and had a pile ov fun too...