I originally had planned to use my factory '78 2bbl carb. It was looking a little rough though, and I hadn't figured out how to deal with the emissions plumbing with it being under pressure. It was at this time that Ebay again came to the rescue and I got a great deal ($29!) on a new old stock '74 Holley 5200. This carb (a license copy of the Weber 32/36) was basically the same as my '78 except no emissions crud, has removable idle jets on both barrels and a larger secondary venturi (275 cfm vs. 235 cfm).
Though this seems small, according to one of the gurus of turbo-charging, Hugh MacInnes, blow-through set-ups don't need to go larger than the factory normally aspirated (no turbo) version since your forcing more flow via boost pressure. (Draw-through carbs on the other hand are under vacuum and a larger carb may be required). I modified the carb for turbo-charging by removing the choke and sealing the actuator rod hole. The floats were also changed to solid nitrophyl ones that won't collapse. From what I have read on another of my favorite forums (
blowthruturbo), many of the Holley and Weber carbs are machined well enough that sealing the throttle shafts is not necessary. I recently decided not to risk it and installed "air seals". To do this, I drilled small holes through the body of the carb to the shafts and also above the venturi. I then epoxied 1/8" stainless tubing to provide higher pressure air from above the venturi to the shafts, this in turn prevents any air/fuel mixture from leaking out.