I'm on the same adventure! I've got a PSA 10.5 upper/bolt/mag block built AR9 currently, but really wanted one that would use the mags I already have a pile of...
Anyway, I was doing some reading on the New Frontier Armory site and apparently I'm going to need at least a different bolt. Which makes sense after thinking about it because the bottom surface of the bolt has to be able to ride over the magazine.
I have a lower parts kit laying around (found one more I didn't remember). I also have a pistol buffer tube (although I think, well I'm sure, this lower is getting SBR'd after I make sure it works well as a pistol). Dammit I'm going to need another Geissele trigger. Just occurred to me.