Welcome Aboard Scott!
There's a fellow on GB selling Nitrided BC and Bolt combos for the 450b for $185. Nitrided is a nice touch when cleaning time comes around. There are a handful of commercially made receivers on the market, which already have an enlarged ejection port. For a 5.56 sized port, it's not hard to open them up with any number of hand tools. Doing so will not compromise their functionality at 5.56.
Barrel are on GB also, but perhaps a member will read this who bought one of them and now has an extra barrels he can sell you for a reasonable price.
In Theory, there is no difference between theory and practice. In Practice, there is.