Shamus,
This is a whole can of worms. There are those who are happy with their scoped M1As, then there some like me who think there is no appropriate way to mount an optic on an M1A. Here is why.
First, lots of scope mounts won't stay on. This can be solved with the ARMs setup above, or a Brookfield, or a Smith Enterprise. All of them WILL stay in place. Problem is, this doesn't solve some of the inherent problems of the M1A and optics.
Assuming you install correctly, the mount will stay in place. You can zero a scope. It will stay in place. Here is where the problems begin.
1) Cheekweld. there is no way in hell a proper cheekweld can be made on a M1A stock while looking through an optic mounted on any of the above mounts. TOO high. The only solution is to get a stock with adjustable cheekpeice, or add a cheekpeice to the stock. The latter of course makes the irons unusable in an emergency. In any case, the scope is lots higher than the bore. Not an optimul sniper type weapon.
2) Ejection. Brass is ejected through the small space remaining between the scope mount, and stock. If you ejector isn't perfectly tuned you get spent cases jamming the works.
3) Cleaning. The M1A is easy to clean without a scope. But, with a scope there, complications. And removing the scope mount is not an option. The ARMs solution might be an improvement on the one peice Smith unit I had. I don't know as I never had one.
Bottom line, there are some of us who don't think the M1A is suitable for scoping. Obviously there are alternate opinions.
If I were determined to put a scope on mine again I am pretty sure I would get the ARMs setup and a scope with a small and low profile to get as low as I could. The I'd buy a stock with built in adjustable cheekpeice. Springfield Armory has one.
However, I have decided that this rifle is a classic that does not require or improve with optics. For me, the standard traditional mode will remain. I will use a FAL or AR10 if I want a semiauto 308 with a scope.
My two cents. here is my baby.