511, 4.25" stroke, limit the build of an RB to ~650-670hp and for a low deck you can safely go to ~720 with a low hard block fill. Girdles help stabilize the Cradle portion of the block and I know guys are making well over 800HP with their stock block race strokers but I won't comment on that which I don't have particular experience building, (but I've "autopsied" a few.)

Cam choice and head flow are critical to an optimized stroker combo, for optimum reliability don't get hung-up on max HP but you can focus on the broadest flattest torque curve you can build while still hitting reasonably high HP targets. You should decide whether you're going with a standard port motor or a max wedge, the ideal "recipie" will be quite a bit different.

Guys that run into bottom end cap walk issues on stock blocks are seeing this because they are effectively trying to turn a moderate RPM torque motor into a peak RPM HP motor; and the comparatively thin walled stock block (intended to a max of maybe 500 lb/ft of torque) will be pushed beyond its limits. And remember the stresses induced by even moderate or momentary detonation WILL push the block well beyond it's capacity. Reliability is further compromised the higher you raise the peak torque RPM. Race blocks have thicker bores and webs and this addresses/counteracts the higher RPM PEAKS this helps high RPM motors considerably because generally (4 cycle engine dynamics 101) once you pass through the peak torque RPM the wall friction of the rings and piston speed primarily starts to act like a brake on the crankshaft, just like a shoe on a drum. The higher the peak torque RPM the more severe these loads will be. I try build the shortblock for overall broad torque and let the heads do the "horsepower work" above the torque peak.

I believe that a 470 3.90 stroke low deck will have the most HP/cube potential in terms of efficiency, but in terms of bang for the buck the 511 is the way to go, ~10% more cubes inthe same package.