Since my slant six appears to have too much blow-by to make me happy when hot ( the propane engine), and seems to burn a small amount of oil, I want to build a shortblock up and swap it next year.

Is there really an important strength difference between any of them, forged or cast crank? I'm going to be using the head I have now, so what is left is the choice of shortblocks to build. I need to use the truck until it's time to swap, so I'm probably going to just get another motor, and rebuild it, and then do the swap quick.

Anything I should be aware of when it comes to choosing which one to work on? I'm aware of the crank flange size difference and that I must have a pilot bearing. Other than that...???

Thanks for any useful input.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

Yep,you are looking for a 225 block with 5 freeze plugs know as the BH block. Its your choice if you want forged or cast crank,both are equally well built and strong. The cast crank is slightly lighter and revs up a little faster,the forged crank in theory is stronger,but I have never seen anyone break a cast crank. I would use a 68-74 drool tube head though because of better spark plug options.

For whatever reason, I'm under the impression that the forged crank engine has bigger bearings. Is that true? This engine is going to, at times, get run REAL hard, and sometimes have long long pulls of 3000+ rpm at WOT. And, running on propane, I expect it to last a very long time, going lots and lots of miles. Shooting for 300 to 400 thousand miles.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

They are slightly wider. If you are going to tow,I would prob. stick with a forged crank,but if not or only light duty towing the cast crank would be just fine. Even with towing,I bet the cast crank would be fine.But the forged would be good for piece of mind.