The Carter AFB and Rochester 4GC were used on the 1963 & 1964 Cadillacs. There was not specific body style, or setup you would see one or the other used and will find either on any particular Cadillac. There were specific model #'s and differences and have posted information I have collected over the years. This might not be complete, but includes the notes I have on file and welcome any updates.

According to the 1964 to 1969 Master Parts book page 219,  The 1964 Carter AFB replacement carb was the 3903S.  The 1959 to 1963 Carter AFB was the 3480S.  The difference between the 3480S and the 3903S is the 3903S has a mounting block on the left/driver side rear with scew holes to hold the electric kick down switch used on the TH equipped cars.  I have seen the 3480's continued to be used on the 1964's that did not require the switch (62 series and 75 series and commercial chassis that had the older Hydrmatics).

Now what was put on these cars later as they were serviced is a smorgasborg.  I have on hand 3904S, and a couple of SR3656S,...but no 3955S.  If you have an new master parts list it probably shows one of the newer carbs.  This was very common.  For example the 1963 Lo and Hi note horns were larger than the 64 horns, but the 64-69 Master Parts book calls for the 64 style as the replacement for the 63 because they no longer made the 63 style.

Here is a listing of the Carbs I have that were originally used for 1963 & 1964:

1 963 8 390 Carter AFB 3480S 577  (without AC)
1963 8 390 Carter AFB 3481S 577  (with AC)
1963 8 390 Rochester 4-GC 7023030 1122
1963 8 390 Rochester 4-GC 7023031 1122
1964 8 429 Carter AFB 3655S 577  (without AC, used as service replacement carb through 1967) 
1964 8 429 Carter AFB *3656S “AD4” 577 (with AC)
1964 8 429 Rochester 4-GC 7024030 1305
1964 8 429 Rochester 4-GC 7024031 1305

Additional Notes added July 3, 2016

For 1963 the Carter AFB's for cars without AC were 3480S
For 1963 the Carter AFB's for cars with AC were 3481S
The Service (replacement) carb was the 3680S ... at least until sometime in 1967 when the 3903s became the service carb.

For 1964 the Carter AFB's for cars without AC were 3655S
For 1964 the Carter AFB's for cars with AC were 3656S
The Service (replacement) carb was the 3655S

CFM Ratings:

CFM ratings were not listed in the Shop Manuals or Dealer Data Books and Carter was really vague in terms of the Carter AFB which could range from 400 cfm up to 900 cfm from what I have read. 

Rochester 4GC is a bit easier as they only made 3 CFM's during this time period: a 486, 553, and 692 cfm carb. You can compare the venturi and main bore diameters (1 1/8 1 15/32 / 1 7/16 1 11/16) to quickly lock in on the fact it was a 553 CFM carb. Since the specs for 63 and 64 are the same you pretty much assume the carbs are the same CFM rating.

As far as the Carter AFB, I had done some research that let me to believe their carb was a 575cfm. There were tons of model # but looking at later ones with CFM rating seemed to narrow it down to 500 CFM to 575 CFM, with the 575 CFM being the closer rating.

Example Pictures:

Carter AFB Carb:

Gaskets & Shim Installation:
--> for Rochester 4GC it is intake, gasket, insulator, gasket,  carb.
--> for Carter AFB it is intake, gasket, insulator, gasket, shim, carb.
Here are the installation instructions you will find in the shop manual in the Carburetor Removal and Installation section (page 10-11, section 20.b. in 1963 manual; page 8-17, Section 27.b. in 1964 manual). 

Rochester 4GC Pictures:

Misc Pictures:

Here are a couple of pictures showing the 1964 Carter AFB with the Kickdown Switch for theTurbo Hydramatic equipped body styles:

Here is the top and bottom of the Intake to Carburetor Insulator: