There are different balancer between early and late smallblock. It's different from the year they moved the timing tab to the drivers side. It was built in in the timing cover.
Oh shoot- you are correct! Also the pulley bolt pattern is different. I’m sorry for the wrong info.
Anyhow- Napa got me one for my 66’ 273 a few days ago. It was spot on. I did not even think twice because they asked 273 or 273/318?!? I said straight up 273... “okay.”
