Longknife, if you can get any twist cut, I'd personally go for a 70" to 80" twist, .005" to .008" deep and grooves wider than lands - 2:1 if possible.

The slower twists will still stabilize the round balls and shoot with less problems as in leading or stripping.

The 20 bore case will hold a descent powder charge and the more the merrier with large round balls. Faster balls makes for longer point blank range.There is no reason you cannot get upwards of 1,800fps with a 20 bore, maybe more. A point blank of 120 yards would be nice for hunting.