I too have fallen victim to the multiple caliber AR bug. So far I'm in for the 5.56 (no surprise), 300 Blackout (now that suppressors are legal for hunting here), 6.5 Grendel (in the works), and a friend just sent me some 450 Bushmaster brass (so now I have to build that one). I also have an AR357, which is the ballistic equivalent to the 357 Maximum round. Here's the developement story.
http://www.weaponsguild.com/forum/index.php?topic=28383.0 (Closer to 35 Remington ballistics I believe.)
The 45/70 may prove to be problematic due to the rim diameter. Rimmed cartridges don't feed well from straight magazines. Especially rounds with a large rim. If you want the 45/70 punch, look at the 458 Socom or 450 Bushmaster. Or even the 50 Beowulf. All run fine in the AR15 platform and the rifle handles them well. (The shooter may be a different story as I hear recoil is 'noticeable'.)
The 50 BMG uppers and others are single shot actions that use the AR lower for FCG and buttstock. 45 Colt would be very interesting.