I have few lowers sitting in my safe (both pre and post 2013). If I have to buy a PSA 300 Blackout pistol kit, can I use any of the post-2013 lowers? and can I pick 7 inch or 8.5 inch barrel length? Will it be MD legal, or do I need some stamp at some point. Sorry, have not been keeping up with the developments of 300 builds and MD laws since 2013. Any advice is appreciated.