todds - we show Holley model 1904 list number 1915 used on 1959 GMC 305 V-6. The Holley literature doesn't show it for 1960, but it easily could be original. If you view the carburetor such that the word Holley on the bowl is visible, look to your left on the boss of the fuel inlet, and if you have a strong magnifying glass, and a pair of "young eyes" you will see the list number stamped.

Personally, I would suggest replacing both intake and carburetor with the 2 barrel Stromberg if you have them available. If not, parts are available for the Holley.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop