If you can find them, Tamiya had a couple 1/25th scale versions of some of their 1/35th scale WWII US soldiers. They could be converted, but as I say, you'd have to find them. I had the one taken from the US Infantry ETO kit of the BAR gunner marching. I can't remember what other figures they had, but I know they had a couple. A fair amount of modification would be required. The M16 might be a problem if you're not up to the scratch building.
You might also check out companies like Pegaso and Andrea. They, among others, have produced several Vietnam figures in 90mm and 70mm. I think 70-80mm is what you need IIR.