Which 273?

brian6pac said

I think the 65 has 5/16" bolts so 1/2" heads on intake, 66 has 3/8" bolts on intake 9/16" heads.
Very easy to figure out.

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Exactly, the block could have been cast 6 months or more before the car was built. If it has the 9/16" headed bolts that bolt in at 90 degrees to the intake surface you have the 66 and newer engine.