64 and 65 had a different set of heads and intake than the 66 up 273 did. The early ones were 5/16 bolts and the 66 up was 3/8 bolts and they were drilled at different angles also.
If it has one shifter mount it is A body. If it has 2 mounts it is for B/E body.
Thanks for the link 4spdragtop ...... will make for interesting reading ....... DTM