UPDATE!
It's been a while since I last posted - damn you Civilization V!!
Anyways, I'm still looking for a suitable deathrolla for the front, so instead I finished up the back, and the "drive wheel" mechanism that pushes this monster around.
The smaller wheel is from the same old kit as the larger wheels. But making one from scratch wouldn't be hard if you really wanted to make one. I used some legs from the killa kan set as a pair of arms to hold the drive wheel in place. I added some warhammer shields to the wheel on each side, and drilled a hole straight through it for a copper rod axle to hold it all together.
I glued this to an engine housing made from plastic card, and decked it out with plates and rivets. I liked the little engine room model, so I mounted it on top, and added a little railing around it for some extra character. (This entire piece is still removable - like the main wheels - for easier painting later)
The two ramp doors were left off, and instead covered with random plates and bits for detail such as an imperial guard flamer fuel tank bit and the engine to a Tau burst cannon (on the side not shown).
I also started hammering out the main gun arm. It's based on a killa kan arm, on which I mounted the original weapon, a killa kan gun and rokkit, as well as a random gatling gun I found. I'll try to add more to it - the idea is to make this arm a mess of guns and ammo magazines.
More to come soon!