The EP-5 is the one with the "ball" of wires that connect the motors to the command board. There isn't room in the GG1.
This is a picture of an unmodified cab interior from Lionel
Cab inserts sitting inside Williams shell
Close up of insert sitting inside shell
You can see the "posts" inside the shell to keep it from slipping down to far over the metal frame. It might be possible to get the height to match by using spacers but I prefer to attach the cab to the inside of the shell. It would be easier than trying to align holes for chassis mounting.
This is the rear cab
modified to fit
I flush cut off the mounting posts and "nibbled" a notch in the floor. The Shell fit over the cab without problems. I will have to do other more serious mods to front cab and/or adjust the mount for the sound board and/or slide it forward. I'm going to scratch build the main control panel. I'd like to be able to see across the cab when done. I still may cut that floor section in the middle completely out and just mount the cab "halves" on the left and right sides of shell. It might actually be easier.