My original has 4 barrels: 219 Donaldson Wasp (barreled back in the old days when the sky was the limit on pressure); 6mm/30-30; 25-35; and 32-20. Anything in a 30-30 headsize or smaller and less than say 45,000 psi should be good.

If it was me I'd start with the 30-30 and then stick with everything that has the same rim diameter.  I have a 44 1/2 that was redone by CPA before I acquired it and my plan is to barrel it up in 303 Savage (I am a fan of the cartridge) and then either move on to a 32-40 or a 38-55 as the rims are so close that I dont think it would be a problem using the same extractor.  I have a Plan B that may swap out the 32-40 and/or 38-55 with wildcats of the same bore diameters but based on the 303 Savage case or even necking it down to 25.  But that's another thought for another day and maybe a good quick swift kick in the head!