Just ordered a pair of LWRC M6A2 16" uppers. I want to put one on a A2 lower. It comes with a buffer and spring, and I am assuming all I have to do is swap a rifle length buffer spring, Correct? I have built DI and Adams piston guns using VLTOR A5 recoil systems, no experience with LWRC and mixing/matching.

Shouldnt be an issue.

Not an issue. I run an LWRC M6A2 SPR upper on an old DPMS lower with a Magpul UBR at work.

Don't think there's anything super special about the spring, the buffer should be an H2.

An LWRC M6A2 should run very well with a rifle receiver extension, rifle spring, and rifle buffer. Do not use the H2 buffer and carbine spring in any receiver extension other than the carbine length, unless you use a correctly sized spacer to make the internal length the same as a carbine receiver extension. Using the carbine sized H2 buffer without a spacer in a rifle receiver extension would mean the buffer not limiting bolt carrier travel, and possibly the bolt carrier slamming into the top of the lower receiver, and/or the charging handle.

I believe the complete Vltor A5 system (A5 receiver extension, A5 buffer, and rifle spring) would work well with LWRC uppers. You might have to experiment with the best weight A5 buffer.