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The 273 and 318A used EXACTLY the same crankshaft and rods. It is right that the 273 piston weighs less than a 318. To keep using the 318 crankshaft without modification, Mopar engineers spec'd a heavy wristpin. The bobweight of a 273 and 318 is exactly the same.

An early A/LA connecting rod weighs 723 grams. The 645 rod (also 496) weighs 758. A decent modern rod would weigh 600 grams. The piston/pin combo is also 100 or more grams overweight.

Take off the weight and port the heads and it'll act completely different, rev much faster, accelerate the car faster, etc. For this I'm talking custom forgings.

One other thing - if you're married to the 3 5/8" bore, keep in mind that the smaller diameter means you can run more compression on the same fuel.

Whatever you do, get some squish into the chamber.

R.

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