hm well first off, unless harris now offers models for the m1a, you will need to add a swivel stud if you want a harris type of bipod.
I have a fiberglass stock and first you have to remove the swing swivel to install a swivel stud. It will require harris' adaptor #2 or #2R, depending on how you plan to reinforce the stock. I used the model #2R which I installed as far as possible from the end of the stock to have as much support as possible as the stock gets thinner by the end. I reinforced the area with epoxy. The screw provided with the adaptor was too long so I hacksawed some of it off otherwise it would get stuck in the gaz system.
In the end I wasn't satisfied with the stability so went back to sandbags, I gave the adaptor to someone here, he mgiht chime in, I can't rememebr his name