Hi Dave, I'm assuming you are working with a factory 70 shifter ? , if so, is the switch popping out of the clip, or is the clip popping out of the shifter ?
If the clip is popping out of the shifter when you put it in reverse, did you install the clip from the lever side or the switch side of the shifter ? The assembly drawings show it going in from the outside of the shifter, but every original car I've disassembled has the clip installed from the lever side where it can't pop out of the shifter. Make sure the tab on the clip is in the larger groove in the tab on the shifter, and the slot in the clip lines up with the smaller, rounded groove. Then install the switch with the rib in the slot from the outside of the shifter tab and push the switch all the way in. The first time you put the car in reverse (gently) the switch should adjust itself.
If the clip is installed correctly and the switch still pops out of the clip, try slightly bending the tabs on the clip that align with the teeth in the switch...
Don't try to bent the tab on the shifter the clip goes in as the previous poster suggested, there's no reason to, and unless your king kong you won't bend it with your finger anyway ( I think he's working with an aftermarket switch/shifter setup)
I'll try and post some pic's....... John