I am looking to put a Holley 600 carb. in my 81 but I don't know what Holley # to put in there. How do I find this out?

 

if you mean the size of the carb. than a 600 is perfect.

 

I mean like model number, aren't they different? How do I know which one.

 

THE HOLLEY 600 DOUBLE PUMP CARB IS ALSO KNOW AS THE 4150 MODEL#HLY-O-4776C. THE 600 VACUME SECONDARIES IS ALSO KNOWN AS THE 4160 MODEL#HLY-O-1850S.THAT MODEL IS FOR THE SILVER FINISH. THEY ALSO HAVE THE ORIGINAL COPPER FINISH.
I WOULD USE THE 4150. IT IS A LITTLE BIT BETTER FOR PURE POWER AND EASIER TO ADJUST. THE 4160 IS A LITTLE BIT LESS EXPENSIVE BUT ALSO LESS VERSITILE.
THAT SIZE CARB IS PERFECT FOR JUST ABOUT ANY SIZE PORT, EVEN TURBO.
I HOPE THAT HELPS.