Jason:
If you feel up to a scratch build and want a big bore, try www.thegunworks.com (or gunworks.com?). Go to the section on barrels and you will note that Joe will even build you a four-bore barrel if you want it. I am no scratch builder, so I am more dependent on kits such as those offered by Pecatonica River (www.longrifles-pr.com). Other folks like Track of the Wolf for kits and parts, and there are many other options.
Good luck, and tell us what you hope to build.

P.S. Shoot that .72 yet?