Have you thought about making it as an actual train, on rails and everything? Who's to say there isn't a railway running through your battlefield?

I always fancied the idea of a 40K Ork army with diesel and steam engines as trukks and battlewagons, except with tires instead of rail wheels.  Could definitely be achieved in Epic I reckon! N gauge locos are probably perfect.

Check out Nexus's Da Steama for the pinnacle of Orky locomotiveness.  Fitting considering W. Heath Robinson is the father of Ork design.

I wouldn't chop a Colossus up to make a Cyclops.  Magnetic barrel extension and a magnetic top turret would let you swap the two out if you were inclined.  Good way to dual purpose a hard to get model.

Here is the conversion gurdil35 did on Epic_fr for his land train.

Mainly bases on DRM miniatures and plastic from this box:


More pics of his nice Squat army on his blog.

Here's a rather crappy pic of the APC turret. Basically it is just a plate with a hatch and gun. The plate is covered in small reactive armor plates and at the rear there's a small box for radio equipment.