They make single shot adaptors for the 788. If you are gonna rebarrel anyway you could do any length,most any profile,any twist,tight neck, . . . AND . . . 22-250AI.
After some info gathering I would most likely go with a 1:8.5 or 9 twist.
Using the 788 action would be a fun project and most likely the birth of a extremely accurate gun. --- SAWMAN