Well first off, cost and how it was designed can be argued by everyone either for or against but I have personally installed the Sadlak Airborne Steel mount on three of my own M1As, a 10 year old loaded model, a 30 year old standard model, and a 6 year old SOCOM 16 and the fit was flawless in every case. The Sadlak mount has a strong reputation for quality as does the Bassett, each has it's own advantage and disadvantage but there will always be situations where the receiver doesn't match the receiver that the mount was designed to work on.
There are a couple of tricks to getting any mount to fit the best it can. Specifically, you need to tighten the screws by hand in a Round-Robbin manner, slowly and rock the mount back and forth until you can't tighten it any more. This helps prevent the screws from fighting against each other after they have been tightened. Then you finish tightening in accordance with the manufacturers instructions.
While you tighten everything you need to pay attention to the alignment of the mount as compared to the centerline of the bore. I use two small bubble levels and put one on the receiver and one on the mount and as I tighten everything down I match the bubbles. The mount's centerline needs to be level with the bore line and directly over the top of the bore line when looking from above the mount down toward the bore. The two places I put the level on the receiver are along the rail by the chamber opening (for fore and aft leveling) and then on the flat behind the rear sight (for left/ right leveling).
My guess is that fore and aft alignment will be off. That kind of misalignment can be caused by the mount not fitting the slots on the right of the receiver properly or a bent mount. One of the things that a lot of people overlook when they install these mounts, regardless of the brand, is the screw hole in the receiver not being properly prepared. You need to chase the threads out and ensure that they are clean and dry. Any crud in there will cause the screw to move under recoil. The slot fit is the key to a tight fitting mount that doesn't move. Sadlak can provide a fit test kit to help determine if your mount will work without special fitting.