Cadet - my bro is currently working with a Joseph Lang 16 bore rifle with a 48" twist. It loves round ball and 3 to 4 drams. I would for-go the heavy slug and shoot round ball.

The use of this seemingly fairly fast twist was common in England in the mid 1800's in muzzleloaders and earlier ctg. guns.

A cupped wad made of plastic or card/fiber will hold an undersized ball in the middle of the bore and spin it perfectly, with zero leading. If using a plastic base cup, you need a hard card wad between it and the powder.

With 2 1/2" paper or plastic, you should be able to use 3 to 4 drams quite nicely. If using brass cases, you will likely need 11 bore wads to fill those cases, along with a somewhat larger ball.

I do not think I would use a ball larger than about .724".
Please consider what I said about a cloth, 12oz denim or light canvas patched undersized ball in a brass case.

Brass case mouths will have to be annealed before crimping.

Jeff Tanner of the UK will make any size mould you want.
www.trackofthewolf.com has all the shotshell wads from circle-fly, listed in actual thousandths with the bore sizes.

Good luck and keep us posted.