"...my Remington 788..." What chambering? Decidedly under rated rifles in any chambering. Rumour has it that Remington discontinued 'em because they were too good for an entry level rifle.
"...Maybe I am being an idiot..." Hi. Not even a little. A guy who thinks about it is a very long way from the guy who just starts cutting.
"...what kind of a price am I looking at?..." Assorted Rem 788 stocks(walnut, et al) can be had here for reasonable money. The chambering matters though. About half way down.
http://www.gun-parts.com/remingtonstocks/
When you install a pad, you can adjust the LOP to fit you. A quick way of finding your LOP is to put a yardstick in your bent elbow and grasping it. Where your finger comes is your LOP. Shouldering a yard stick like a rifle and looking where your finger comes works too.
Yes, you must cut the stock to compensate for the thickness of the pad. A mitre box(can't say as I've ever seen one wide enough for a butt stock for any rifle though) or saw is the best way. Followed by a bench belt sander(your stationary sander) to match the pad to the stock. Masking tape on the wood helps.
"...if I messed up mine would be a nightmare..." Only if you don't have the tools. Not everybody does. You have tools/machines most guys don't. Especially guys living in one bedroom apartments. I'm insanely jealous. snicker.
"...There are better ways to check..." Nope. Far more complicated for something that isn't.