It'd be more work to modify the tonner chassis than to find a good ute or van chassis. It can be done though, but you'll need the back half of a ute or van chassis. Cut the tonner section off the front half of the sedan/wagon/coupe subframe at the front (the front bit is the same on all body styles) and join the ute/van rear half on. Ute/van rear half would need to be dressed in the reverse of the tonner chassis's prep. Best bet is to try to look at a sedan/wagon/coupe chassis side by side with a commercial chassis and you'll see what I mean.