I just looked in my Brownells cataloge and they sell an adapter of some kind for "Ruger 77 mkII plastic stocks". The part number is 416-102-101. This may be what you need for a Harris bipod but you should call them to make sure cause they don't give a picture or a more detailed description of it.

Here's the link to it on their web site. http://www.brownells.com/aspx/NS/sto...&s=17803#17803

Or you could still just screw in a swivel stud and mount a versa pod that has another built in swivel on it.