I don't think it will be any less cumbersome in the field on your 39 than it was on your 45-70. I mounted a Marbles Tang sight on my son's ancient 39 because it was drilled and tapped for it already and I figured he'd probably like it for target shooting. If he gets tired of it, it will come off and be boxed as one of those ideas that didn't pan out. Personally, I find them an annoyance. They are in the way whether in use or folded. Receiver sights are much better.
I don't think Lyman says one way or another in their literature if the sight for the big bore Marlin was the same for the Model 39. I went with the Marbles for that reason.~Andrew