My thought would be that you should mock things up before welding final brackets and mounts in place. You've got the obvious concerns regarding getting the motor back far enough while not back too far, getting it high enough while not getting it too high, and making sure headers, fan, linkage will all work.
Then you also need to get the motor level left and right and the crankshaft centerline aligned correctly.
Once you know where you want the motor you can determine what will be the best way to actually mount it.
