Jaz, I can't help with your question beyond the good advice of others who suggested having one made up exactly like you want it.

I know of at least nine different bore diameters that go by "8-bore" in 3.25" and there're probably many more, so it may help to determine the land/groove diameter of your blanks and if you plan for a thin brass or paper case sized chamber, then get someone with a similar gun to get you dimensons from a chamber cast. There are a lot of 8-bore guys here and I imagine someone has something similar to what you have in mind if you have to go the custom reamer route. I've got an 8 shotgun, ball and shot gun, and rifle and would be glad to loan a casting if it would help.

Bob