First off, I want to apologize ahead of time if I’m doing any of this wrong.

Ok, I am trying to put together a precision rifle from my AR pistol lower. I’ve decided to put a 6.5 Grendel complete upper on it with a FAB Defense RAP stock. I chose a 20 inch heavy stainless barrel, rifle length gas system and has the .750 gas block. My question is where to start with the buffer tube, spring and buffer?

Either.

Rifle tube, spring, and buffer. Or carbine tube, with suggestion of standard carbine spring and H2 buffer.

OP, do you have a buffer tube or receiver extension yet?

If not, I'd recommend the rifle receiver extension.  Std rifle buffer should be fine.  I would recommend the Geissele super42 rifle spring, but a std spring would work