I think you'd be happier with the M16 conversion. It doesn't flop the case back into the action after hitting the scope as often as the others. Some smiths claim they can do it correctly. If incorrect, you'll need a new bolt or put up with the stoppages. JMHO
The 721 doesn't use a riveted extractor! They are a snap in similar to the new 700 models. They have a hole at each end to hook into to remove or help to install. Try not to bend or spring it anymore than necessary to install. Usually removing the ejector will give you a bit easier spot to drop the one end into with less distortion of the new extractor. Squeeze it a bit and insert where it will go in the easiest then carefully rotate into place. Kinda hard to explain on these vs the newer 700 ones as not many get replaced anymore. I have standard and small but no mags anymore.