If the scope tube is too short, you need extension rings.
If it is just a height problem, you just need higher ring or rings. Every M77 I have had uses 2 different ring heights, the taller goes in the back. I think they come with a medium/high. You could probably just buy one ring (the next highest size) or email ruger. They are helpful with such questions and can tell you what you need. I would just buy a pair of Warne highs and call it good.