Do you just want to do a Vietnam version. You can use the M151A2 and do some crazy stuff the marines did with them after the war before they had Humvees, even after they got them.
They used rollcages and all sorts of weapons and mods, very cool. They needed a light attack vehicle and had little money to buy them so they came up with that. The roll cages looked like plumbing stock and not like the race cars. Real low budget stuff.
As for Nam. To get it right it's not exactly about the armor. They had a lot of different versions, like hillbilly armor until kits started showing up there. They did use some of the M151A1s that mounted the RR gun with it removed. But most were plain old M151A1s.
So that is something you can play around with. It's the arms they used. It's really about getting that right, how they mounted them and what they used.
I have seen a gunjeep with a tri-mounted M60 arrangement on a pedestal mount and a lot of double M60s. The best one yet and I am going to make this one very soon I hope. This one is really cool.
It had a mini-gun in front of the passenger with the ammo in a tank behind him. In the back on the pedestal mount was one of those hand-cranked 40mm grenade launchers. That is the coolest M151A1 I ever saw. You can find it on the web you just have to do some google searching.
So yes you can use a lot of leeway just get some of the important stuff right. A lot of them had an M60 on a short mount in front of the passenger. Very few had a 50 cal, they were too powerful. Some mounted the 50 cal on a tripod in the back and fired only forward, probably with the driver out of the jeep.