I might be the "Montana guy" who had a 760 .30-06 rebored to .35 Whelen.

Picked it up at the Missoula gun show already rebored by JES. The bore also looked great through my Hawkeye borescope, and it shot as accurately a .35 Whelen needs to shoot for killing big game, around 1" to 1-1/2" for 3 shots at 100 yards.

As I recall the rifle was made in the late 50's.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck