I made this using a GHQ M1083 FMTV chassis (GHQ #N129) with the bed from a HEMTT (Ghq #N95) and then mounted a .50 cal on the roof and two M249 SAWs in the bed along with an 81mm mortar for a little extra boom.  Scraps of sheet metal were added around the rear of the bed to protect the mortar crew.