These are extremely old kits. They were, I believe, originally sold separately; the idea of packaging them as a pair came later. They're on different scales. The new Lindberg website (bravo for its honesty) says the Monitor is on 1/210 and the Merrimac/Merrimack/Virginia is on 1/300. I think I remember buying the set in one of its earlier incarnations - without the vacform base, but with an electric motor for the Confederate ship. (The Monitor didn't have room for one.)
They really aren't too bad for their day (the early fifties). To my eye the turret of the Monitor looks a little undersized, but at least it's in the right place. (The competition of the era, Pyro, put the turret of its Monitor forward of amidships.) I haven't actually had my hands on either kit in many years, but the overall proportions look reasonable.
I'm unaware of any aftermarket detail sets designed specifically for these kits - or any other Civil War ships. You might be able to make some generic parts (rail stanchions, gratings, etc.) from Gold Medal or Tom's Modelworks fit. If you drill them out, and clean them up with a file, they shouldn't look too bad.
If you do a Forum search on tne name of either ship you'll find some interesting stuff. Good luck.
I don't think you're going to find any aftermarket guns, either. Bear in mind, though, that only the muzzles will be visible. (As a matter of fact, the current thinking among the experts seems to be that the Monitor's two guns were set up in such a way that only one of them was projecting through its port at any given moment; the recoil of one ran the other one out. So to depict that ship accurately you only need one gun.)
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