Mounting a scope on an M1A is a potentially frustrating experience. Most aftermarket M1A or M14 receivers and even some military spec ones have some significant dimensions....inconsistencies that can make mounting a scope hit or miss. I've got a Springfield Armory, Inc. M1A Supermatch with a scope mounted via a McCann mount. It's a good solid mount, but it requires some gunsmithing to fit up, nothing you can't do yourself with care, but you need to drill the block that mounts in the clip guide dovetail. That block slides in free and is tightened to the mount body, then you need to tighten it into the dovetail and drill it for the springpin. The mount may not actually align perfectly with the bore. So you need to have plenty of adjustment in the rings or in the scope itself to get the windage dialed in.
Also, keep in mind that this rifle design was never really intended to have optics mounted and the mounting point is something of an afterthought rather than a precision mounting set up. The Springfield inc. mounts, the McCann and probably the ARMS and Brookfield mounts and its derivitives may all need to be shot in to settle things in properly. The McCann mount's instructions specifically tell you to hammer on the mounting screw for the main mount as you tighten it up, using a 2x4 as a buffer. this shock sort of swages the threads of the bolt to the receiver and allows you to get a tighter fit. It also ensures that the tenons on the mount set fully into the mortises in the receiver. Once this pounding is done the mount should be set and it can be shot, unmounted and remounted with minimal shift in zero (less than 1/2 MOA in the case of mine). My SA inc. mount required significant shooting in before it began to hold its zero, and all bets were off if you dismounted the thing and then remounted it. Maybe 2-4 MOA.
This is true of all the standard 1 and 2 point mounts including the Brookfield, ARMS, Smith Enterprises and other mounts. Some of the newer mounts have totally different ways of setting things up and I'm not at all acquainted with them, so I can't speak to them.
Have fun.
Buy the best scope mount you can find and mount it carefully, then expect to have to fiddle with it a bit to get things zeroed. Once everything is dialed in and settled in, it should shoot well, though you may want to elevate your cheek to get proper eye relief and a good cheek weld.
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