I have a 1975 CJ5 with an AMC 360 engine. Just bought it not too long ago and the wiring job from the previous owner is horrible. I took it out this Saturday and just about had an electrical fire on my hands, so I figure now is as good a time as any to do the rewire.
Probably have to build a plate to cover the hole and mount the EZ wire box away from firewall slightly and cut a round hole like you have to run them through. At least to where you have the grommet so it's not a strain on the wires at the box. I used the EZ wire kit for my early bronco and was happy with it good luck keep pics coming.
Hey quick question regarding your post...which one did you buy? 12 or 21 circuit. I also have a 75' CJ5 and have the identical problem. And dread doing any type of wiring. And did you just connect all the wires from the engine compartment straight into the fuse panel? Because I know I have some that go into the back of the fuse panel and some that go straight from the firewall to other things...
I'd cut a nice square piece to fit over the original bolts in the firewall. Then cut the appropriate sized hole for the grommet out of the middle. Cut it across the middle so it can be bolted on, and have the grommet in the middle. Problem solved.
Beat it to fit, paint it to match!