Well, the time may come where 7.62 x 51, surplus or otherwise, may not be available. In fact, we had a form of that for almost a year already. What will you do, or better, what will I do, since I have half a dozen of that caliber rifle.
The difference isn't the size of the cartridge, it's the chamber of the particular rifle. 7.65 x 51, has a thicker brass case and the shoulder is often "annealed" so that the cartridge will tolerate more headspace. Thus a particular rifle will be useful longer. The first thing to do is check an older military rifle for 308 headspace, using a 308 guage. This is a good idea anyway, when you happen to buy a surplus rifle. At some pont you may want to handload, although some munufacturers don't recommend it. If your military rifle is on the long side of headspace, the brass is going to grow to fit the chamber. I the same way, my MAS 49/56, converted to 308, functions well, but from what I hear, will distort the brass to make it unusable for other 30 caliber rifles. It's a pain but segregating brass may be necessary. Anyway, if at some point you have to use 308, and it may come to that, use lighter loads and lighter bullets. The FMJBT from PRVI Partisan seems to be what would work, but I haven't fired it in those rifles I'm not sure about. It is also as vulnerable as any other imported ammunition to import bans, so that may not be a viable option either. So, since there is Lithuanian, German, and South African 7.62 x 51 available now, the best option is to stock up now, and avoid the hassle later. There is always white box Winchester, but how easy would it be to run THAT out, or for the government to persuade them not to sell it? I only worry about the MAS 49/56 and the Argentine FN-49, and not my FAL or M1-A. I had the FAL checked, and the headspace is good, in fact, at the shorter end of headspace.