Hmm. As it happens I've just modeled an old rhino I bought at a charity shop (£1.50!!) into a assaultback- though I'm not going to stick the top panel so I can switch it out for a rhino.
As for the modelling aspect it was actually a piece of cake! The kit I got was pretty old (not sure if it's up to date) but it came with a slot for a razorback, even thought it didn't have the guns themselves. The turret slot uses the same turret size as a Land raider, so fitting the twin ACs from a crusader came easy.
Az