I use #7 Heavyweight Flitecontrol 3" in my 20ga H&R TDC for Turkey, but it's a modern barrel, #5 or #6 lead should work fine in the vintage barrel for turkey tho as long as it will pattern acceptably, you'll have to determine the max range of effective patterns tho, but like I said, make sure the chamber is made for 3", the 1947 barrel is likely only good for the shorter rounds, it's always possible the chamber was reamed to accept the longer shells tho. If it will accept 3", Federal makes lead Flitecontol loads too, I shoot the 2¾" 12ga #6 in my 1940 H&R.
If you want a rear sight, you'll have to either have it drilled and tapped or you can epoxy it on, some have done that. There are front sights that can be affixed over the bead, some are magnetic.
Tim