I have been reading about M14/M1A from other gun forums, but i get mixed reviews about every maker of M14, who saying saying the truth
If you want to use a USGI scope mount, most of the newer receivers are out of spec and will need to be machined to your receiver for a correct fit.
SAI receivers are commonly found out of spec. If you use a Sadlak mount they will provide pins to determine how to correctly machine a mount for your rifle. Sparrowhawk mentions the same thing for his dummy selector switches. Mine is out of spec.