Most shops and mail order companies have spacers for a few bucks, usually in 1/4" thicknesses, that can be used to lengthen the stock a bit, but 1 1/2" is a bit much for that.
The Trap stock w/ Monte Carlo is longer, mine runs 15", while the utility stock on my parts gun runs 13 1/2", and an friend's 870 Express splits the difference at 14".
A thicker pad and a couple of spacers might give you enough length, but if it's a "Serious" shotgun, you may do better leaving it short.
BTW, if you go the above route, you'll need longer pad screws.
Or.....
Leave your pad on it, add a coupla shims made from scrap leather inside a slip on pad and put it together. It won't look as nice as a Purdy, but it'll work fine.