David - I think the best thing to do at the outset is to get a barrel blank made from quality steel - preferrablyone rated for centerfire rifles. You didn't say if you were going to bore/rifle your own, or buy an already rifled blank. If you are buying a blank (Green Mountain has a big selection) that already has the lands and grooves cut in it, and just threading it for a breech plug - you are essentially doing what gunsmiths do every day when they re-barrel.

If I was using a top quality MZ barrel, like Green Mountain, I wouldn't even worry about proofing it - unless you were turning it down so thin that wall thickness was a concern. About the only other thing that could bite you in the keyster would be if the threading of the barrel for breech plug was done improperly - such that the only a portion of the threads were holding. Then the breech plug could wind up coming back into your right cheekbone But otherwise this doesn't sound like a real complicated project - or at least a lot less complicated than re-barreling a centerfire rifle.