I have not seen a 760 marked 760R, but they are stamped Carbine. What is the barrel length? Can you post some pictures? If it is a cut off rifle, as I suspect, then the value is only as a shooter. If you mean the R that has a circle around it after the 760 on the receiver, that is to indicate that 760 is a Remington trademark. There is no record of a 300 Sav. being made in the carbine length, only; 30-06, 270, 280, 308, and 35 Rem.
For a couple of years in the late 1980's Savage marketed a combination gun made by Marocchi of Italy which was chambered in 12ga over .222 Remington and 12ga over .308 Winchester. The gun was designated as their Model 389.
I do not think that they make larger clips for the Savage model 62.I know for certain that they do not make larger clips for the Savage model 64.
A while back there was a long action version of the Savage Model 12 (I think it was the 112BVSS) that was chambered in 7mm Remington Magnum. The last I ever really heard of them was back in the late 1990s. I've not seen anything about it since then. For now the only manufacturer that makes a bull barrel 7mm Remington Magnum is Remington, their Sendero line, as a matter of fact.
Remington Arms has been manufacturing the Model 700 series since 1962.
Yes, I have one.
No
Remington does make a combo shotgun rifle, the Model SPR94.
Timney makes a great aftermarket trigger for the Remington model 700,that will fit your Remington model 722 rifle.
Remington didn't make a model 54, Winchester did but not in 22 (that I know of), Marlin model 54 is a shotgun.
No winchester makes a model 1300 shotgun.
yes
Yes, in 1965, Savage made the model 99C. It was available in the 7mm-08, and is valued higher than the other calibers on this model.