Whoa!
Guys, you gotta chill!
My guess is the reason you aren't getting an answer... is NONE OF US KNOWS!
Your 3.5" 870's are actually VERY uncommon, compared to 3" 870's (probably the majority of those made). But among older Wingmasters, lots of them are 2.75" ONLY.
And most of the people who do have 3.5" 870's likely have not modified them.
My suggestion is to simply buy and modify. Get it in your hand, compare the magpul forend to the forend you just took off your 3.5", and see what differences there are. I mean, you are only risking $24...
EDIT:
oh and look at that, someone had success in the post above before I could post my reply!