700 aics 338 vs 338 cip?

Either should fit standard 700 screw spacing. As long as your action inlet is as large or larger than CIP length then I would go that route. (I can measure my CIP chassis if you need)
 
If you are just putting an CIP AI stock on your long action rem no, you will not have to modify your action. It will fit fine. CIP is longer. If you are gonna use it for the 338 edge or the 300rum, you will have to mill the 2 side cuts out of the mag.