I know the first is gonna sound weird but:
First, I'd ditch the skinny tires and canabilize some tires from the marine bikes. If thats cost prohibitive then something else that has meaty gnarly fat tires with treads that look like steel plates. Turning them into half-tracks would be cool also.
Second, there looks to be too much in the way of fuel cans and crates on them. I know they are intended as a convoy but somethings off. I think its the lack of a dedicated cargo space.
Third, to fix the second, pull the cargo beds off of the diecast models and put them on the more angular cars?
If you don't want to do that how about scavenging a couple of deuce and half models for the beds.
Fourth, Convert some of the packs or boxes into radios with antennas or some kind of obvious communcation equipment.
Fifth, I like the armored plate concept. Have you considered do more of them?
If so, how do you think ones that come to a point in the center would look?
Sixth, KingCracker had a point about the roundness. If you make the armored plate come to a point in the front it gives you the chance to make the front fenders angular by either using the old rear ones(If you use the cargo bed or half-track ideas)or scratch building from plastcard.
Seventh, Convert some weary/wounded troops riding on them. Give them running boards for those "special scenarios" where you might want to have troops hanging off the side(HHmmm, drive by's
These are just a few ideas. I hope there is something in there that you'll find interesting or useful.