The bore depends on what pistons are available. Most people over-bore to clean up the cylinders, so the minimum required for that is wise. It does raise displacement slightly but if you thin the cylinder walls too much you get problems with distortion under pressure and such. The designers weren't stupid. If you want to push the bore, you should have the wall thicknesses measured w/ ultra-sound first.
Don't over-carb. As mentioned, the critical thing is the size of the primaries, not the max cfm w/ secondaries flowing. The Thermoquad and Quadrajet were the best design, with small primaries and monster secondaries - termed "spread-bore", but I know of nothing similar for smaller engines like the slant. With fuel injection, you can get similar combinations. There is a smaller flow version of the Holley Commander 950 (600 cfm I recall). If still not small enough, you might install smaller injectors. It flows only 2 injectors until you mash the pedal. Haven't heard of anyone trying one on a slant.