I did a simular build using a Merkel 12ga. As far as the barrels go you just have to bite the bullet and pay the $200 I used Douglas. I thought about the .405 but went with the 450-400 instead.
Same barrels and the reamer was the same price, but just seems like more cool factor. I regulated with Hornady factory ammo (I can't see irons well so I added a scope in Talley lever lock rings) and after I finshed regulation I loaded some 300gr bullets and found the barrels regulation held (approx. 4" spread at 100yds.) but the sight regulation was off 12" but that was easy to ajust with the scope. The 300gr. bullet at about 2300fps proved to be enough gun for my whitetail last seaon. The only problem with the 12ga. frame is the gun is heavy 11+ lbs.