Interesting project. Hope it works out for you.
You may want to consider buying used barrels instead of making them, unless you've got your heart set on doing it. A good source for used barrels is ebay or Gun Parts Corp.
If you have your heart set on making barrels, you still might want to buy one for a rifling guide.(You can make a guide by pouring lead around the guide rod while inside the barrel).
For barrel steel, google for seamless DOM tubing. 3140 or 4140 is a good type of steel for barrels. The heavier the wall the better.
Or in lieu of all that, you can buy a rifled liner from Brownells, which can be epoxied in a suitable piece of steel.

For the frame and other parts, it will be easiest to use thin sheet metal for a template to get the parts working together, as well as for future reference.
I've thought about making one of these as a muzzle-loader in order to legally by-pass barrel restrictions, but haven't done it.

How ever you do it, please keep us posted.