A forged 360 Mag piston that comes in around 8- 8.5:1?

Stroke it to 408CI and use these off the shelf pistons.

Icon Pistons IC982-030 Fits SB Chrysler Dish 34cc Bore 4.030

Icon Piston IC982 (specifically designed for boost) has a 34cc dish for use with a 360 running a 4” stroke.
This will net 8.8 to 1 CR with 65cc heads.
I believe this is the lowest CR possible with an off the shelf piston.