I've made several MF (Magnum Force) level pumper guns and in order to help maintain accuracy in these high pressure situations I used BKL double strap scope mounts. They worked good but as can be seen from the picture below the stock Crosman steel breech has a 1/2" space for a sight.
The 2 strap BKL mounts are 1" so not really the best solution. The Disco breech (top) has a fully machined dovetail and I'll use them for my future builds but in the meantime these BKL 3 strap 1 1/2" mounts will bridge the gap very nicely. They have 3 straps and 4 screws as opposed to 2x2 to hold the mount to the rail. Cost $23.95 each from Pyramyd Air.