A used M6 Pedestal Mount, for Humvees, will run about $1200 plus freight.
Humvee M6 MG Pedestal
Since you only have a 5.56, I would suggest getting a UPA Universal Pintle Adapter ($60-80)
Universal Pintle Adapter that adapts that large tripod taper pintle to the straight vehicle pintle. Then get the repro pintle socket about $60
Repro Socket avail from odonod on ebay; Sarco Inc, International Military Antiques, Numrich/Gun Parts Corp) vs $250 for a proper heat-treated pintle socket (
M25 M31 Pintle Socket ), use the correct sized DOIM seamless tubing to make your vertical post welded to piece of diamond plate, then add about 4 each support legs around the bottom made from 1.5" angle iron.
You can get a complete repro WWII Jeep style pedestal, a later M38 Jeep pedestal, M151 etc cheaper instead of the M6 Humvee Pedestal from Sarco.
You can get crazy by getting a Mk64 Mount to drop the M249/pintle into that plugs into the UPA then pedestal or get an M249/M240 cradle with or without swing arm mount that usually have the M249 drum dovetail along with a 7.62 NATO ammo can holder
M249/M240 Cradle
For my War Pig gun truck I found the original mfg for the co-driver LMTV GPMG Mount, Military Systems Group, who hooked me up with a swing arm and cradle,
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