Mike,
You may be interested to know that Bob Wallack did the original write-up of the .243 in the Sept. '55 Rifleman. His ad in the same issue included the line "Rifle Rebarreling Specialist", from his shop in Northville , NY.
I'd shoot it before tearing it apart. It doesn't look like it was initially built as a mountain rifle anyway-the scope is a giveaway.
If you don't like the cartridge, the .240 PSP will clean up the chamber.
Good shooting.