Lots of different bores and strokes make up these different engines. You can get most of the aftermarket blocks in 4.125" bore and long what seems like 4.25" stroke cranks these days. Rod length is also a limiting factor to some of the combinations as you start getting into higher RPM ranges. Also heads may not work really well for some big cube stroker kits so be aware of things like that and other components such as cams. The larger cube motors do have some nice trade offs since you have Cubes, you don't have to rely on a peaky high compression (and racing gas) and winding motor. Cams that may be too radical for a 351 might be mild for a 427. The big stroke motors are easy to run on the street and make a lot of HP and Torque too. Side loading gets bad with long stroke short rod combinations. As the stroke gets longer to maintain good rod ratio gets difficult with some combinations. The worse are ones that don't use custom parts and may be a bad compromise at best.

I think the better question would be the ideal rod to stroke ratio rather than bore to stroke. Its the rod ratio that will determine how much piston side loading you will have. Ripper is correct is that piston side loading increases when a long stroke is used with a short rod. In the past, a decent rod ratio would not exceed 1.55-1.58 (rod length divided by stroke). However, with good forged pistons, a strong block and good rods, you will probably be fine with 1.4-1.5 range. I spoke to noted race engine builder Bennett Racing and this is what Jon Bennett told me, "When building a long stroke 351 4.125-4.25 stroke, we base the rod length soley on piston design. Example: If we are building a turbo/blower engine with 4.125-4.250 crank the shortest piston we can use would be 1.300 due to dish requirements & rod BE thickness, so we would be forced to accept a 6.050-6.070 rod. Nitrous or no power adder engines the longest rod we would use would be 6.200-6.250."