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.
Remington Arms has been manufacturing the Model 700 series since 1962.
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.
No
Yes, I have one.
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.