The problem I had with the M1A was that even though the mount went straight on, it was so high, I couldn't get a comfortable shooting position. I had to lean way into it, and it kept banging me over the eyebrow. (Only fun the first few times.) I did a little digging, and realized that to really use it as intended, I would have to get a different stock, with a raised or adjustable comb. This changed the budget of the project significantly.
Also, the sight picture was actually obscured by the mount. The whole point of trying the project was that I could still use the iron sights, but when I had to move up the rear to zero, I ran out of space. I wound up selling the project to someone more serious than me, and getting another Remington. If I were to try it again, I would probably look at the AR-10. I checked in Impact a few years ago, they had a DPMS that started at $800 something.