josh,
Far as I know, no one loads reduced recoil slugs in 20 ga. Maybe soon someone will start, since there are some good 20 ga. home defense shotguns coming on the market, but not right now that i know of.
Adding weight to the gun is the first and easiest way to cut down on felt recoil. Birdshot in the stock bolt hole is an old time and easy fix, as Dr. Dave suggested. And it works, too. You can hog out some excess wood in the forearm to make room for lead there too if need be.
A really good recoil pad will help a lot also, again as Dave pointed out. It's also a good idea to make sure the gun fits you properly- shortening the stock to one degree or another helps some folks, adding length helps the longer/taller/lankier people. Gunstocks are made to fit 'average' users, some folks just aren't average.
You might be able to do quite well using the barrel you have, which I assume has a bead sight. You just have to figue out where it shoots with a given sight picture. No way to do that but to put rounds downrange, I'm afraid.
I'd suggest shooting from a standing position with a good solid rest for your support hand. Shooting a shotgun with slugs from a bench is a good shortcut to a flinch for a lot of people, and you don't need to develop a flinch. Start with a target at 25 yards, if that works well back it up to 50, then 75, then 100 yards. Learn what those ranges look like well enough to estimate them in the woods, and once you know where your gun (and you) shoot at a given range, you'll know if you should attempt a shot at that range.
Out about 100 yards with a bead-sighted gun you will most likely be hitting the wall as far as range is concerned. Could be you can't confidently take a shot at that range, only you can answer that question and only then with experience on the range punching big holes in paper targets.
Try a variety of rifled slugs (not a lot of reason to spend $$ on sabots for a smoothbore) and see if your gun likes any brand/load better than the others. All this is going to call for a good bit of shooting- don't rush the process, don't overdo it and crank in a flinch in the process. Spread it out, take your time, use good form and concentrate on your trigger pull and sight picture. Above all, FOLLOW THROUGH just like when shooting a rifle. It can work, but it will take work on your part to make it happen.
And if you can afford a slug barrel for that gun, it might not be a bad investment. A rifled bore for sabots, sights plus a little extra weight with the 'heavy' barrel can't hurt. See
http://www.hr1871.com/Support/accessoryProgram.aspx for info on barrels available.
hth,
lpl/nc