Go for it! I have a 3 inch 65-2 round butt that stated out as a 4 inch square. Work done by Mike Larocca with bead blasted finish, stippled back strap and trigger job to boot.
Go for it, a somewhat easy job. I doubt if a 4” model 13 will be a high demand collectible. If you do it, carry it and enjoy it. Have a model 15 that is now a 3” full lug in .327. Here’s pics with different grips, Larry
I have a frame wrench with a k/l frame insert that I'd be willing to loan you for shipping costs.
Is the gun done yet?? The gun in post 19 was round butted by me with a hack saw and file. Made it fit a set of round butt grips. Larry