Look for one that states that it's a drop in replacement. I think Wyatt's is, maybe PT&G but I'm not sure, theirs may be the same as an M5 footprint.
Most of them require a little stock shaping to get them in. If this is a hunting rifle I'd recommend the H-S Precision version and get someone to fit it up. They sell mags that have under 5 capacity, so you get the convenience of a box mag w/o having it hanging out the bottom of the rifle to snag stuff, and face it, how many times have you ever made more than 3 shots on deer. I put one in my 7-08 hunting rig and really like it.