Questions/Doubts Re: 1966 273 Commando Intake Manifold

Is that about where the old road-draft tube was?

I don't believe any 273 ever had a road draft tube. Different makes put the road draft tube in different places. However, Mopars with road draft tubes typically attached on the valve cover where the pcv was attached when they started being used.

E.g., this pic of a 56 Plymouth chassis shows the road draft tube hooked to the rear of the passenger side valve cover:

chassis.jpg