what in the sam hill do these do? Power stacks?

Yup, my 67 Fairlane (CA car so... smog)had the air pump and injectors on the exhaust ports. You can see the threaded holes the air injector rails screwed into just below the valve cover lip in the photo below.

View attachment 1715982546
I see they have the extra exhaust mannyfold bolt holes. 390GT? 428?