Beginner Question: How to tell 273LA from 318?

JBurch said

64, and 65 273's, intake bolts are 5 /16 inch and will take a 1/2 inch wrench/socket to remove/tighten; intakes are single plane, both 2 and 4 bbl; 66 and later 273's intake bolts are 3/8 inch and will take 9/16 inch wrench/socket to remove/tighten, intakes are still single plane, 2 and 4 bbl.

a big inspection mirror on a stick would help to read casting number on block.

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Juz sayin but that only tells you what HEADS (and maybe intake) are on the short block. To show you how far astray you can be, in the early 70's I "dumped" a good set of 273 heads/ intake on a THREE SIXTY short block. It was a torque monster up to about 4K RPM and then you might as well shift---or turn off the key, because it didn't get any better LOLOL