My pre-64 M70 300 H&H came to me used with an aftermarket shotgun style pad that was shot. I replaced it with the period correct pad linked to here. Though not an exact repro of the original, it's darn close and is the only pad I'd consider for a pre-64.
ANY pad that is unoriginal to the pre 64 model 70 depreciates value. The 300H&H rifles never had pads on them originally. At that point, you might as well glass bed it like another poster said because you or someone previously turned a $400+ stock into a $150 stock. The repro's do not mitigate recoil as well as the Pachmayr handwerk suggested either. The red pachmayr also looks great. There is not real positive by going to the repro..
I've heard those repros are stiffer than a wedding prick.
Igloo, they are not quite that hard. They do offer some recoil absorption, but not nearly as much as the Pachmayr I'm suggesting:
I use these on Winchester model 70's, Ruger 77's and anything else that needs a recoil pad. I like the red color, as it goes very well with a wood stock or brown painted stock. There's no reason to install a repro pad, unless you think you are trying to convince yourself or others that is is original. Just be aware, someone like myself will come along and call you on it. Most pre 64's, as you know, did not come with a recoil pad. As they had steel, aluminum, or composite butt plates. Some models did come with ventilated recoil pads and the solid red pad, but those models are few and far between.
Like I said earlier, once you replace the original pad/plate and cut the stock, you have depreciated the value of the stock and rifle big time. On a pre 64, they are slightly curved and also have an inletting on the top for the widows peak butt plate, unless manufactured after '58-59. The op can do as he wants, it's his rifle, but will likely not find an exact fit pad. A grind to fit is about the only option.