After the fresh air box is removed, and the linkage, how do those boxes/control valves that go up into the defroster difusers come off? I know part is that metal ring to squeeze, but not sure what all holds it in?

There are bolts with flat, wide heads which hold the base plates to the body. The vent pieces mate with the base plates and are held together by the spring clip.

So I would need to first, remove the bolts from under the dash, and then the two parts would separate?

So no need to remove the spring clips necessarily?

Using this picture as an example.
https://www.ebay.com/itm/1974-Porsche-914-R...ewAAOSw8lpZG7Y8

Where the oval part enters through the dash, clips hold it on to a base plate?

Go under the dash. Look for the two 8mm nuts on each vent. Remove them. Then you can caregully remove the air boxes and defrosters. You can remove the front clip that holds the defroster to the air box but tha back one is near impossible to get to. Try not to break the tabs held together by the round clips.

I was able to install mine assembled by carefully maneuvering them into position. I had dis-connected the 2 vertical braces that hold the cowling rigid so i could leverage the cowl up very slightly. Good luck.

This site was very helpful in my re-assembly:
https://914electric.wordpress.com/2011/03/18/take-the-heat/

Barefoot