Originally Posted by Ozarkdude

I'm the wrong one to ask about transmission swaps. As far as the intake, right behind the filter they welded a panel throttle plate/butterfly. Remove the manifold and using an air powered die grinder, cut away the entire plate so the engine receives unrestricted air as it was originally designed. You may want to consider retaining the upper shaft and vacuum controls to retain its "appearance". If it were a later car with a trap or cat, maybe you could do something to open up the exhaust, too.

While ive never seen behind the filter on the 603, or peered into any of the 60X turbo motors, I understand there is similar vacuum controlled airflow blockage/EGR that could be removed. Remember, this blocking (closing the butterfly) occurs at low/part throttle positions (cruise), which is why they appear to make almost as much peak power as their euro counterparts. Those throttle positions are exactly where a diesel is its most efficient, and exactly where the system is blocking the most air while feeding it exhaust.