Does anyone know of a bipod that fits the pre-99 Ruger synthetic stock? Most bipods attach to the front sling post, but these stocks have an integral swivel

Just go buy an Uncle Mikes swivel stud, drill it and screw it into your stock either in front or behind the swivel that's there now depending on how much room you have up there. Preferably in front of the existing swivel.
Problem solved for about $4.00

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.