In theory, smaller calibers require a faster twist than larger ones, so the twist shouldn't be a problem, but as mentioned above, getting a good fit with your ball and patch might take some doing. I've found that "rules" can be pretty pretty flexible when dealing with MLs, like sabotted bullets shooting pretty well in 1-48" barrels where supposedly they shouldn't. Won't cost much to try.
You may find .45 maxi-balls around, but stuff other than .50 cal is scarce in stores. The Lee REAL bullet is another popular slug, but you'll have to cast those yourself most likely. Your sabots may do just fine, but I'd prefer Barnes all-copper bullets for serious work myself.
Good luck!
