[tlorber]

You do not HAVE to set the motor back this far, and in some applications you may not be able to (i.e. if the body is sitting very low on the frame) without modifying the firewall, which I did not need to do. However I think the real estate is more valuable in front of the engine than behind it in order to provide as much clearance for the fan/radiator and other accessories. Setting it back also distributes more weight to the rear, as these trucks tend to be front heavy. What I have been seeing here is that there is really no "right" or "wrong" way, just ways that you can see that others have tried successfully.