[Larry Fulton]

According to the NHRA Engine Guides for '69 & '70 Chevrolet, these are the correct sizes and specs

'69 & '70 Chevrolet Passenger Car

- with 350" SBChevy / 2bbl. Rochester

- 2GV Model (divorced choke) with

1.686 is the Throttle Blade Bore ( 1 11/16" ( 1.6875" ) with

1.375 is the Venturi / Bowl Section Bore Size ( 1 3/8" )

(these "Big Venturi" 2bbls, are the one's that all of the dirt racers wanted for years.... sometimes incorrectly referred to as the 500 cfm 2bbl. )
(note the '71 Engine Guide shows the SBChevy 350" - 2bbl. carb had been downsized to the 1 1/4" Venturi)

** NHRA Engine Guides for '71, '72, '73 & '74 Chevrolet's - Document the SBChevy 400" - 2bbl. as having the following carb specs.

1.686 is the Throttle Blade Bore ( 1 1/16" ( 1.6875" ) with only a

1.250 is the Venturi / Bowl Section Bore Size ( 1 1/4" )

Hope this helps!