i have plastic bushey . bought stag 3gh upper I have changed trigger scope and installed single point sling. That is the limit of my experience. my plan is strip bushey and install in cmmg then rebuild bushey to have two complete guns. I can buy complete cmmg for 225.00
But in case you're saying you want to put an alloy Stag or CMMG upper on the fiber lower - DON'T!
I'd leave the BM alone and just build another AR if that's what you want to do. Why bother stripping the BM then turn around and stuff different parts in it? Doesn't make any sense, especially if you ever sell it - who wants a bubba'd C15.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek