have you calculated your compression ratio with these slugs? they're listed at 8:1 which by my very scientific WAG probably puts them in the hole a fair bit. that negates a lot of the benefits of your closed chamber head and the quench it (theoretically) brings to the table.
but cut the heads and run a thin headgasket and you can claw back some compression, and you're maybe sitting at 9:1 which, with that 292 cam is gonna be straight dog water.
orrrr... you zero deck the block and use headgaskets to dial in your compression ratio with the piston of your choosing.
orrrr.... you cough up for good high compression pistons from the jump. which, *pulls chance card* advance to 440 source and pay for a stroker instead.
anyway, i think you're shooting yourself in the foot here by using a heavy cast replacement piston in this application. if you were building a tow rig, or something that was in a light car and you wanted a torque monster that was all done by 5K it'd be a different tune i'd be a whistling.