You are correct, it is 5/8x24 from the factory. If you're able to find an Armalite brake secondhand, it would certainly be an interesting option. I did consider either that or a Sako TRG brake, but found that they're more difficult to find than the AP, T3s, Badgers, etc. Again, just remember to factor in the crush washer as that's how the Savage brakes are indexed from the factory... If you're looking for a metal-metal mating surface, you may need to have some material removed from either the barrel or the brake itself. Not a deal-breaker by any means, but something to keep in mind.

I've not any experience with the Muscle Brake, but I've heard that they're very good as well. The APA Fat ******* may be another option to consider, but it sounds like you're a pretty big fan of the Armalite piece.