The BREN 805 has ample room to make the 7.62x51 work...BREN in 308 would be....amazing.
BREN 2 in 308 would be game changing...or...*shhh*, 6.5 Creedmore....
Actually, I think converting an 805 to 6.5 grendel would make the most sense. Heavier receiver and no last shot bolt hold open in particular mean from weapon balance and speed of operation, an 805 is better suited to a stand off weapon w/ a longer barrel. 6.5 Grendel is also an option, but from cost per round, ballistic performance, and optimal barrel length comparisons (w/in 2" of one another, starting at 20-22" for Grendel and 22-24" for CM), the Creedmoor is much superior at all ranges and minimal penalties relative to Grendel. So if you're not restrained to a 5.56 magwell, then go creedmoor over grendel.
Actually, building the 805s into designated marksmen rifles and 806 into carbines seems like a no brainer, but uncertain about whether parts, armory, etc, logistics make that more imprudent than first blush analysis.
I look at the 805 as a rifle and the 806 as a carbine. For a carbine w/ 12-16" barrel, I like 7.62x39, 5.56, 6.8 spc, and .300 blk, and in that order. YMMV.