I've built several 3.91 stroke RB motors, I'm almost positive that KB, Icon brand, makes three different shelf pistons for this combination using a 7.100 long BB Chevy forged H beam rod , dish, reverse dome, (half a dish) and flat top with valve reliefs so you can get the compression ratio you want scope that combination rocks in all configurations thumbs

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)