Since the 721 has been out of production since 1962 I doubt many if any stock suppliers have that action to verify the inletting. From what I know the action screws are on the same centers but I don’t know about length. How the action screws line up with the bolt handle, safety and trigger I can’t say. I suspect that any stock would require some inletting work.
I would stay away from any aluminum bedding blocks. Wood could probably be made to work. As for the Archangel, I would be concerned about the magazine figment and just how much work might have to be done.
Whatever you decide, I would contact the fabricator first.
I gave a little more thought and I would suggest you look at Boyds (they list the 721). Synthetic stocks (polymer or plastic) really didn't become a item until the mid 60's when the M16 came out and the M14's were having warpage issues. Most firearm suppliers didn't start with synthetics until the late 80's other than on items like the Nylon 66 22 LR.
I just used a Remington 700 BDL short action stock on the 722 that I re-barreled to 6mm/223, and converted it to a 722 BDL.. I think that if a 700 stock will work on a 722, a 700 long action should work on a 721. To use the BDL you will need the 700 bottom metal. If you use a 700 ADL stock the 721 bottom metal will require inletting. There is a slight difference on the bolt handle slot but that is only in looks. I also agree with Doom above that Boyds can probably supply a 721 stock, and they do a good job.
With detailed fitting at the safety lever, and adjusting the barrel channel, a 700 stock will go on a 721, 722, and 725. Also the floorplate needs to then be the 700 plate.
I have fit 700 stocks on two 721 and one on a 725. The 725 Enfield type safe lever was quite a challenge. It required removing and adding small wood pieces.
Here is a 700 stock safe area adjusted for a 721 safe. This is a 721 rebarreled to 280 by Larry Brace.