Carb CFM guess

they have several cfm sizes/ some are 325/365/425 and 485 each gm division had a different size carb for engine the 1 11/16 /3/8 venturi is rare and only came on the 400 and up 2 barrel cars / and hd 350 engines on school buses dump trucks ,and some 350 engines with 3&4 speed cars