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

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
I want vintage parts, its part of the theme of my Dart, Offenhauser is a Good as it gets