Thermoquad to square-bore adapter mods

You do have a wrong adapter. You have one for a Quadrajet. There is a difference in adapters.
Look up the the thread- Square - bore vs. Spread - bore? by Bill Crowell on Feb. 3 2021. I explained the differences and posted a bunch of pictures of the same. You need Edelbrock 2691 for a TQ, I know, the picture is identical to the 2692 Q-Jet model and then the one you have is the Q-Jet also. You will understand when you see the comparison pictures, noting the area in the rear between the two bolt holes.