Hey Chris
Yes the give in the cage is due to the slot being cut which opens up the back end all the way to the loading port. Also there is no need to cut a channel down the side of the 870 reciever in order to get it to fit in either after you do this.
DATA is the best guy to talk to about this really and I'm sure he'll chime in. He did the G/L section on my PR for me. I did it myself originally and the results were OK but not great. There is only so professional you can be when your doing it on your living room floor after all LOL
Things to be careful of are undercutting the barrel and cocking tube, mainly the tube which holds the shells though as if you cut it a little short (like I did the first time) then it screws everything up. Same applied with the racking arms.
As everyone always says it is better to cut too little off than too much. You can always shorten them but making stuff longer again ain't so easy.
This is why my PR now is actually using it's second Maruzen 870
Cheers
Darren