I have a 7" barrel and want to attach a 3 lug adapter (with a AAC Evolution 9MM suppressor) while shooting subsonics, would that kill me? Obviously not supersonic.
If you are going to go that route(of just using a piston with fixed barrel spacer) why not just buy one in the correct thread from AAC since 6 or 7 threads are available?
Unless he didnt charge you hardly anything for the work, I would rather just have one from AAC in case there were any issues. I could see AAC denying warranty coverage for using a modified part if you had an issue like a baffle strike.
As far as I know, AAC doesn't make a piston with a 5/8-24 thread pitch, probably because they don't want people using TiRant 45 suppressors on 300BLK since they aren't rated for the use of supersonic ammo on that caliber. AAC does not recommend using the TiRant cans on 300BLK in case someone accidentally runs supersonic ammo through it and damages the can, themselves, etc.