The best way to make the side rails is to just use one of the plastic ones to transfer the screw holes. I made all mine out of 1/8 x 3/4 aluminum. Pick up a stick from lowes. If your hands are large like mine, they'll thank you for using the thinner stuff. Trust me!
The rear plug is if memory serves me .987-.990 dia./ .750 long / 10-32 on the sides and .250 top to bottom. Mind you I haven't made a stock adapter in many years but I made many, someone correct me if I'm wrong.
Rambo