Jaybird
If your 56 is like my 55 you are stuck with that 4GC. I have one on a low milage 55 (19k) and after being apart 8 times, swapping parts from other (parts carbs) it still runs bad...stumbles just off idle. On mine the exhaust cross-over in the intake manifold is directed into the base of the carb. Unless you get another manifold, that is the only carb that will work. Otherwise the Carter WCFB is a good carb and easy to set up. Other than the manifold you will need to change or re-direct the choke heat tube. Adapters and spacers usually create more problems.
Sorry for no help...you probably knew all this.
Willie
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55 Centurys 63; 63; 66C
People who use tools bleed a lot!
Keep your mouth open so it doesn't hit you in the face!
BuickRestorer