Welcome to the furom,
The heads are on the big side as far as port volume goes. Those heads would be better suited on a bigger cubic inch motor like 540 and up. I would take the 1 inch open spacer off and just bolt the carb straight to the manifold, or put a 1 inch 4 hole spacer on, something to increase torque and your carb signal.
Your heads are killing your torque, its a classic case of bigger is not always better senario.
You might want to consider stroking your motor out to a 496( 4.25) or a 540 (4.50) if you every intend on getting any power out of them high flowing heads. Or you could just get some smaller heads.
Goodluck, Charles