I think that if I owned one of these things, I would keep my powders on the fast side in order to avoid port-pressure problems.
Brass can be made from .250 Savagee, .22-250, .300 Savage, .308 and about 40 others..... only problems might be with neck thickness so you might have to ream them out....... actually, if you had a whole big bunch lying around, this would be an easy conversion from .280/.30 British!
Keep powders on the fast side, bullets on the light side. Internal case capacity is close to the 7.62x39, external ballistics are identical. You should be able to work from the books this way.
Do have lotsa fun!