I know this type of question has been asked a million times before, but I can't find anyone talking about this specific combination, so here goes.

First time building an LR-308, and I want to be 100% sure that my upper and lower are going to match without a sledgehammer / sand paper. Both Aero Precision and Fulton Armory say their parts are DPMS pattern, but does that mean they will work together? Do I need to worry about a Gen I vs Gen II compatibility issue?

Parts in question are Fulton Side-cocking upper
http://www.fulton-armory.com/upperreceivertitanslick-sideslottedforsidecockingmodification.aspx

and Aero Precision lower
http://www.brownells.com/rifle-parts/receiver-parts/receivers/lower-receivers/308-ar-m5-lower-receiver-prod71464.aspx

Thanks guys for your help

Both are DPMS pattern so they'll fit together.  

The only possible issue is cosmetic. You may not like the way the lines of the upper and lower match together but that's about it.

Also, the upper is a high-profile so make sure you get the right handguard.

Other then that, I checked out the upper and side cocking BCG.  That's cool stuff man.