Stick, welcome to Nitro Express! An 8 bore is a wonderful thing, welcome to the club.

You didn't say if this one is a Paradox or fully rifled; I am guessing a fully rifled?
If JJ slugged the bores then almost assuredly they are .838"/.839" groove which would indicate a rifle originally chambered for paper case ammo. (brass case rifles were usually .875" groove)

The projectiles may or may not fit in the modern plastic 8 bore cases at that diameter and be able to chamber, so your choice would then be custom brass cases from Rocky Mountain or go with an .835" ball/conical which was the usual paper case bullet diameter. The lubed wads will prevent gas blow by and the rifling will still be able to grab the ball.

I would try to get a sample .839" slug to see if it will fit the cases as well as chamber then mould cut at equal to or max .001" over the larger side, so .840" if that will fit.

Look up Cal Pappas (Wasilla), he's the Alaska guru on bore rifles and can likely help you out.

- Mike