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.