The '64-5 models generally used the 315 casting....the '66-7 models, the 920 casting, and the '68-9 models, the 275 casting. The 315's and 920's, with good high octane fuel, perform well on a 273. Their drawback is crack problems and pump gas ping problems. The 275 casting is open chambered and doesn't have any particular crack problems so it's better suited for a pump gas daily driver. Use the '72-4 version and you get hard seats too! The 273 and LA318 used the same castings in the same year but there were some crossover stuff that is the exception to the rule. I've had 315 castings machined from the factory for the '66 intake and 920 castings with the '65 machining! Maybe different plants got the new castings at different times and just kept using them until they ran out? Very weird....
