I'd go with the .530" balls, Doug, considering the smooth bores. That gives .005" compression per side and should be just fine. I don't think tighter than that is good, as you might have to go even thinner on the patch.

As to loads, I'd start at 2 drams - about 55gr., 1 gr. per calibre. This will give VERY low pressures and, I feel, a good place to start with an original.

Hopefully, all will go well and you can incrimentally raise the round ball loads to about 65gr to 75gr. 2F. regulation will tell, if it does regulate.

Nice looking gun.

Did you see the 13 bore straight rifled single W.R.?