WTK info on fitting the CSS Internal Receiver block

[Chiroone]

I am attempting to fit the CSS internal receiver block to my S12. I have cut off the tang on the rear trunion and have attempted to install the internal receiver block . However, when i attempt to mount the receiver block in the holes for the former original FCG, there is about a 1/16 gap between the rear of the receiver and the receiver block. Any one else have this problem? I am wondering if I have to egg out the holes in the receiver to make it fit flush? Any one have any thoughts on this?