My dad had a 1938 Belgian Browning with a fixed modified choke. A great 2 3/4" shell is the Win XX 2 3/4-1.5oz. -#6 ( 1 5/8oz.) if you can find them . Also try the Fiocchi Golden Pheasant in 2 3/4-1 3/8- #6 or 7.5. The Federal flite control in 2 3/4,1.5- #6s if the others don't work. Full or as full a sthat choke goes should do it for 30- 35 yards . 40 yds is a stretch and really needs to be verified before taking that long of a shot with those chokes.
The steel in that 1948 gun is as good as they make it. It is the damascus or twist steel barrel that you have to be very careful about. As far as an occasional shot of HTL in that barrel it would not be that harmful. It is the Lyman choke that I would be checking on.