I wouldn't use factory BDL c.o.a.l. constraints as a limiting factor for building a rifle/bullet combo around! There is a spacer in the rear of the mag box that can be removed, replaced, or otherwise modified to accommodate longer rounds.
Whomever is building your rifle should be able to do that work, if needed. If not, there are prolly some DIY threads online that could guide you along?
I like that you're determining what bullet you want to shoot, and building your rifle around it, smart move! Settle on your bullet choice(s) and match the barrel's twist & chamber's freebore to suit. With a little time & effort, you should be able to run long c.o.a.l. rounds from a modified BDL mag box. Or, switch to an AICS based DBM, and you can run 2.520" c.o.a.l. rounds from an AI .223 magazine...