I've built three longrangers for friends along the lines that you are going. A .338-416 Rigby Imp (same as your choice but NO BELT), a 300 Ultramag and a 416 Chey-Tac.
All of them had 1.45" dia. Lilja barrels and the alum blocks were made two-piece, top and bottom and were in HBRs from McMillan. The barrel blocks were 11" and 12" long, depending on action length. I've done several more but the big alum block was dropped into an aluminum stock instead of an HBR. One of these shot a 4.9", 10 shot group at Missoula, last Sunday, 6.5-284 caliber.
Drop me an email if you want more info on block options.
Jay, Idaho firstname.lastname@example.org