Heres a conversion I did awhile ago, using the rhino from the Blood Angels battleforce, the assault cannon from the Land Raider Crusader that I destroyed for a trerrain piece, and a bit of elbow grease.
Here is how it came out, unpainted So far:
Now, how did I do it? Well, the first thing you will need, if you have them lying around, around:
A saw and file, I used these, specifically:
And after you get these, you'll need an assault cannon. I used the bits from the LRC, and hacked off the ammo chain thing it had on the bottom. Alon with that, I sawed down the inside edges of the two cannons to allow them to fit over item three, specifically the turret mount from the Imperial Bastion kit. Making sure they fit snuggly over the center bar, as this will serve as the anchor to keep the guns on the razorback. Apply some glue and you'll get something like this:
And that is ready to fit on the Razorback weapon mount roof option from the Rhino/Razorback kit. Congratulations! You now have an effective,WYSIWYG, and, if you have tons of buts lying around like I do, cheap, conversion for an awesome vehicle.