The model 742 magazine will fit and work perfectly in a 760 - except that the mag follower will stop the bolt from closing on an empty chamber after the magazine empties.
The bolt may be closed, using the 742 mag, via either manually depressing the follower while closing the bolt, or removing the empty mag before closing.
BTW - I spotted a .300 Savage-marked Remington magazine in a scrounge box, while I was looking for a set of used pistol grips a few days ago.
I can't recall if it was for a 760 or a 742, but I'm sure it could be had cheaply.
Here's the shop's website, w/contact info: