Say I want to have a maple stock LOP shortened by an inch and a half.  Who's the go-to to cut it and reshape the butt pad to match the new profile?

Any wood shop can,,look for guitar makers, modelers, etc.

I've been wanting to do this with a 20 gauge o/u . My length of pull is 133/4 and my o/u is over 14 inches. I'm afraid of messing up the gun but it's not an expensive gun .

A decently equipped gunsmith can easily cut the stock and sand the pad to fit the new butt or replace the pad if the OEM style cannot be trimmed inward.   They use a sander with a jig to follow the outline of the stock including the toe angle.

I have done the above by hand and it is a bit tedious.  I made a saw box to hold my taped stock (to prevent splinters and tear out)  and cut with a fine back cut saw with very little to no pressure especially on the end of the cut.   Screw on pad, scribe outline.  Remove from stock. Cut excess leaving enough at the toe to continue the line of the stock.   Sand with a block until you get to the final dimension, checking frequently against the stock.

Edit, I forgot I also scored with a utility blade, my cut line all the way around the stock to reduce the chance of tear out.

Google “Cotton Branch”

Josh specializes is sbs. He can do anything you want on your shotgun.