The only thing wrong with a Marlin Guide gun is that stupid hammer block safety. That thing can screw you.
A bit of file work to the offending end and a drop of Locktite as well as a twist to the set screw was all it took to eliminate the hammer block problem.
The tenoned inletting on the Marlin would certainly be stronger than the 870’s butt attachment. However, nothing says “can’t lose this” like the stock on that M77, and a M77 Mk II is a pretty tough rifle to better in a bolt gun. It’s not quite as handy as I’d like it to be to suit my preferences, but it’s a much better deal than the typical H&H or 458 type in generally as-issued form.