Are you talking about a "real" Iron Duke, or the basic GM 2.5 liter. There is a BIG difference. A true Iron Duke is a Pontiac 151 from the early 60's. It was literally a Pontiac 301 with the drivers side cylinder bank cut off and a plate put over it.
Then there was a 153 Chevy II that some people called an Iron Duke. It was basically the same block as the standard 2.5 liter, but the head was different. The exhaust and intake were on the same side. I believe the Distributor location was different also.
Then came the standard 2.5 that came in the Mazda's and Sunbird's in the 70's and early 80's. It was a cross flow head. Intake and exhaust on opposite sides. This is the most common configuration. I'm not sure if the newer Fuel injected versions have changed much, but, I raced the carb versions for a few years and depending on how much your allowed to do you can really get some HP out of them. The pistons are the same as Chevy 350 pistons and I know you can use 350 rods also, but, there may be some machine work required if I remember correctly. I had Crower 5.7 rods, J&E Pistons, and a 703 lift roller cam in mine. You can work the heads quite a bit too, mine had 2.02 valves in it. If your lucky enough to find a marine application, it will have a steel crank.
