I'm not sure why 700s wouldn't proof for steel shot. The only difference between steel shot proof and Superior proof is that the proof loads for steel shot proof contain steel shot. Not a question of increased pressure or anything like that.

And OK with me that the single trigger differences are "apt" to be minor . . . but we don't know that without taking a look at the "guts" of both guns. There's a very good photo of 400 parts on p. 67 of the Crawford and Whatley book. That'd be a good place to start if someone can come up with a 700 parts photo.