Danny, I was all stuck on the 1972 dual carbs with that one-year-only vacuum source off the right brake booster elbow.
You, however, remain stuck in vacuum hose limbo.
Do you know where the correct vacuum source nipple for the preheater thermostatic valve resides?
Diagram 2 shows it for the 1973-74 engines. It is on the central idling plumbing under the air filter, it is a little vacuum nipple next to the big pipe (blocked off) that used to go to the air pump.
Is it capped? Is it welded shut? Is it just open and sucking in air god forbid?
It is your vacuum source so you can do the mixture reference test!
I need to know how this missed your attention so I can redo the write-up and diagram.
The nipple off the right carb only has a little vacuum at mid-throttle for the now blocked-off EGR valves! thus, the nipple needs to be blocked off!)