Holley makes a 650 cfm spreadbore vacuum secondary carburetor that may function like a T quad? I think it may bolt right in place of a t-quad on that spreadbore type intake? Never messed with one, but they have been around for awhile.

Used Edelbrock 600 or Carter 625 would also be fine. I actually like the Carters better, but they are old enough now it's not so easy to find a cosmetically nice looking one. Once you set them up they are very reliable.