I have acquired an old Stevens double barrel (311 series H) It was my grandpas and went to another family member, so I have no intentions of selling it. In fact I intended to use it. Its in 99% perfect condition.
Just got me to thinking, if I have it trimmed for length of pull and put a different butt pad on it. How much will it effect the value?

They are not that collectible of a firearm in the first place, but one heck of a great gun for hunting. Built like a tank and will probably still be around when we are all gone. As for affecting value, probably not a lot, but if you are not planning on selling it, don't worry about it. Make it into something YOU like and will get years of enjoyment out of and then pass it along to someone else.