If you want to use any CB larger then 13T you cant use have the engine screwed directly to the chassis. You will need to use engine mounts. And like eminent said, and I said a few posts back, check the brakes. The disc could be hitting the pads causing that clicking. If your not sure, take the engine out and try rolling the truck back and forth and see what happens. If its smooth then its probably the mesh.
Rob that was a nice find, especially with all those extra wheels. I wouldnt run it with two different tires though. Its going to wreak havoc on the drive train.
With the proper gear mesh you should be able to put a piece of paper between the CB and the spur and feed it through when you roll the truck without tearing the paper. Anything tighter or looser and you are going to eat up the CB