If I had the cash, I'd buy that gun from you just so you won't shoot steel out of it.

Hevi-shot makes some loads called classic doubles that are non toxic and safe to shoot through an old gun but are an arm and a leg. I think I paid like 35$ for 10 or something like that..... But it's better than muffing up an old gun and making a decision you'd regret
That isn't a duck gun. That is a beautiful piece of art that belongs in an upland field chuckar quail and pheasant hunting that gets put in a case whenever it leaves the mantel or your display case. Get yourself a pump, but for the love of guns don't ruin that gun.
My duck and goose gun is a one piece full synthetic stock/receiver with the only bit of exposed metal being the barrel just because if I drop it in the mud or salt marsh it won't get hurt.... If I did that with my side by side, I'd go home instantly to strip it down and make sure there wasn't a single spec of mud on it...
To answer the question of why an old gun shouldn't shoot steel is a very broad and complex question to ask, but here are the basic points on why it is a very bad idea
1) steel shot does not compress like lead shot
2) old barrels might be made with softer steel, and while 1 or 2 shots won't hurt the gun (depending on choke) eventually the shot will wear the barrel down
3) steel shot can't be shot through anything less than a modified choke on a gun that isn't rated for steel because of 1 and 2
4) you have to be bloody insane and or desperate to shoot something if you want to destroy the value of something like that
5) in the off chance it does fail, it is simply not worth the risk of possibly killing yourself if the gun blows up on you