PK, no need to rebarrel a BLR for .243 or 7mm-08. Just go to the shop and get one out of the box. Since they also come in .308 and .358, the only '08 cases missing from the factory rounds are the .260 and .338 Fed (for now).
Personally, I think it is asinine and shows little regard for the safety of others (never mind yourself) to take a rifle designed and proofed to operate at 42,000 p.s.i. (not CUP) and rechamber for a cartridge that operates at 62,000 p.s.i. (also not CUP). I fail to see the thrill in firing what amounts to a proof load every time I touch the trigger, when for about the same price as a Marlin and a gunsmithing bill, you can get a perfectly safe BLR.
Everyone also needs to consider who will be liable when you get tired of your new toy and sell it. Sometime down the road, some non-handloading city dude, who bought it third hand out of the newspaper, is going to drop a couple of sharp pointed factory rounds down that tube magazine and drop the hammer. Guess who his family is going to have the investigator trace down to pay city dude's surgical bills PLUS pain, suffering and attorney's fees (both attorneys)