In my opinion, if you are integrating several of these units into your games you should really make the small investment of at least D12 dice, if not D20’s. This allows for better variance between different units. Personally, I do D20’s. HBG has a spreadsheet on their page offering suggestions on both D12 & D20.
However, in a D6 format I might suggest a few variations to the above mentioned ideas.
Trucks: (as squadron commander pointed out the cost effectiveness issue) I use a cost of 2, moves 2, no attack or defense. Trucks can carry 2 infantry or 1 infantry & 1 artillery. They can be used to bring tools into combat zones and can be taken as casualties.
Air transports: cost 6, move 4, no attack or defense, can deploy 1 paratrooper into combat (subject to AA), or 2 infantry from one friendly territory to another friendly territory.
LVT: cost 5, move 2, attack 2, defend, 2, acts like a reg tank but grants +1 to infantry attack rolls (paired) when used during amphibious attacks.
Jus some ideas……