Keeping with the slim lines of the rifle I wouldn't mount anything with an objective bell larger than 40mm and smaller than that would be preferable. The 1.5x5-20 isn't a bad idea if it fits the ring spacing.
I have one on a LA M70 XTR (7x57) and it fits fine. I'm not sure about the spacing on M77, though. As an aside, from what I understand, Leupold will put together a scope with a gloss finish if you ask them to.