If you take a look at the Mobile Redoubt from A&E Guide pg 56, you'll see that it has a speed of about 10 with clumsy manuverability, AC 3 overall, section hp 100 (Hardness 5) section AC 3, and a ram attack that does 9d6. Though certainly not a tank by modern standards, you could use that as a baseline.
How about this
Hit Points: 100d12 (800)
Initiative: as Driver/Gunner
Speed: 20ft (4 squares) Clumsy, or Burst Speed
Armor Class: 18 (-2 size, +10 Armor Plating)
Base Attack: as Gunner
Attack: Machine Gun +6 ranged (2d6 +5, x3)
Full attack: 4x Machine Guns +6 ranged (2d6 +5, x3)
Special Attacks: Trample, 50in Cannon
Special Qualities: (Not sure what to give a tank)
Burst Speed: 40ft (8 squares) for 1d4 +1 rounds. Must be a straight line.
Armor Plating: The entire tank is covered in sections of hard steel armor plating giving those inside full concealment. Each 5ft section of plating has DR20/-, 10 Hardness, 50 hp, and a break DC of 30
Trample: 9d6 melee?
50in Canon: +2 ranged (5d6 +10) can only be used once every 1d4 rounds
I'm not very good at stating out vehicles but hopefully this is a good start