I know in Carter, Edelbrock and Holley re-carb kits, the float level gauge is included. It's just a cardboard mini-version of a carpenters square, printed in 1/32" graduations. It's L-shaped, 1/4" wide and 2" X 1.5" -- usually.
Since a GM "special tool" of a '63 vintage is extremely rare, I would ask any hotrod buddies who are still into carburetion if they have any extras lying around.

I/I #4101
'71 GMC Jimmy 350, sm465, np205,3.73 posi.
'68 C/10 Stepside 292 (.030 over) Offy Intake, 500 CFM AFB,Clif headers, sm465, 3.73 posi.
'67 K/10 454 project.
'72 K/5 rolling frame project.