65 Dart 273 hi-po. Are these 9 or 10 inch drums?

They’re 10”. Look at the difference in the shoe width between yours and BK Cow Gods 9”. Also, a quick look inside and you’ll find that there are no push rods for the wheel cylinders to push out the shoes, 9” had an embossment right on the shoe that sat right on the piston inside the wheel cylinder. And, the size of the dust cap also gives it away, 9” uses a much smaller dust cap than the 10”