What holley carb do I have?

vent excess fuel?? uhh, no. Try they are there b/c it'sa double pumper carburetor. Sounds like a 4150 HP. (the one with the two "pipes" coming off each end anyway)

1850-2 is your std Holley 600 cfm vacuum secondary carb. The other list # I tried to find on Holleys site also, but their list of list #'s is impossible to read, the print is so small as to be useless. I'll look in my Holley carb book later and see what it is.

9834 is also a 600 cfm vacuum secondary carb. Don't know what the difference is here in between the 1850 and 9834, both have the same description in the Haynes/ Holley book.

The 1850 is the one you want, can't comment on the other, might be an emissions design 600. Just make sure the throttle shaft bushings are tight on the 1850. I'm running one on my V8 Ranger. Bought it used for $125, swapped the fuel bowls for a dual feed set from a list 3310 750cfm Holley. Installed a white vacuum sec spring, left the jets as is. Runs perfect. secondaries open a second or so after you nail the gas. Removed the choke assembly, don't need it down here in Cajun country. Engine it's on is a stock roller 5.0 short block from a wrecked 93 E150 van, bored 30, with hypereutectic flat tops. Cam is a Ford F4TE high torque roller, with the lift improved with Cobra 1.7 roller rockers( 256/266 degree duration .450/.474 lift) . Ported E7TE heads, stock sized valves. Intake is a Ford A321 ( same as the old Shelby Cobra high rise) Headers are Shelby type tri-Y's ( for early Stangs) Lot's of fun with a Toploader 4 speed and 3.73 geared rear in the 3800 lb Ranger, be even better in a lighter Stang like your's.

Does it have one or two metering blocks?

4150 has two metering blocks, 4160's has one block on the primary side, secondary has a metering plate.