Modifications to Offenhauser 1 1/2 carb spacer Thermoquad to Holley pattern ?

You won't see the mods, its all inside.
Well you can make that plate work. You'll need the thin Edelbrock plate to seal the bottom and then you have a lot of grinding ahead of you. But no way would I do "all that". Here's what I'd run. It will work fine.

Summit Racing SUM-G1401 Summit Racing™ Carburetor Adapter Plates | Summit Racing