My wife and I both shoot 20 gauge Remington 870's for hunting and in still target competitions. They are great guns, very reliable and will last a lifetime with just a little care.
You should be able to find barrels pretty easily to fit that gun. One dealer, also an authorized Remington repair station, is Mann & Son (Since 1945). You can reach them at 618.357.2911. They are great folks and John Mann is the best gunsmith that I know.
One thing that we've learned, over the last 8 years as we've worked with the Swarm 20 Gauge Turkey Choke in research and development, is that the 26" barrel has a tremendous advantage over shorter barrels in 20 gauge guns. There's no real advantage to anything longer but when you go to the really short barrels, you sacrifice a good bit in patterning.
Doris and I have been fortunate to have set 2 world records and Doris has also won a world championship with her 870, the Swarm 20 Gauge Turkey Choke and Hevi-13 shotshells. That makes a good combination.
I'd also recommend that you consider the Winchester Xtened Range Hi-Density shells. They are excellent and also do well with the Swarm Choke.
The only lead shell that I can recommend is the Winchester Supreme Turkey shell. It does not have the range that the "hard" shot shells have but will deliver acceptable patterns at reasonable ranges.
If I can be of any help to you whatsover, please do not hesitate to contact me and I'll do all that I can to be of assistance.
"If he's out of range, it just means that he has another day and so do you."