The OPFOR here at JRTC run a lot of civvie "technicals" with MGs mounted in the beds of pickups. One is even a dual M2 .50 cal mount.
Interested to see how they ran them, I looked in the back of one the other day and all I saw was a large plate of steel to which the mount was welded. The plate was almost the interior siz of the bed and had 1 hole at each corner where they secure it with ratchet straps to the factory tie down points in the bed. Then the gunner snaps into a point near the base with a gunner restraint system harness (GRS) for safety while for sh!ts and gigs, they don snow mobile or motocross helmets with stickers that say "Space Shuttle Door Gunner" and other silly stuff.
I'll try and get some pics at some point if it would help.