Unless you have it really hopped up look for something around 400cfm. If you are into the unusual, look on some of the english stuff (Rover, etc.) because they bought the design and use it in several English makes. I have an acquaintance who drives a MGBGT V8 that is the same engine you have.
Like others have posted that carb is a bit too much for that engine. Don't kill a great little powerplant by drowning it and killing the bottom end power. Frank
Buick originally used both a two barrel (the 2-GC) or a four barrel (the 4-GC) Rochester carburetor on the 215. The Oldsmobile version could be had with a large 1 barrel and a turbocharger (the Jetfire option). Pontiac also had their version. There was also an aftermarket dual 2 barrel intake available. The original Rochester, while pricey, is an excellent choice. Jon.
The favorite carb and intake for the Rover/Buick 215cui is the Edelbrock 390 (expensive but the best) and the Offenhauser intake. I had a 600 Holley on a 215 and it drowned it. Fitted the 390 Holley and it ran like a champ
I have a olds 215 with offenhauser 4bbl intake in my 75 Lotus Elite with a 625 cfm Carter carb. Way too much. I am now shopping for that Holley 390. Good luck man.
Funny enough... I have one with that set up. 1981 Rover 3500 with performance intake and exhaust. Just put an edelbrock Performer on it. yeah... lot of carb for the motor, but it's what we had. Need to take it down a few steps I think. Car came with a big Holley double-pumper. At least we are headed the right way. So.. anyone wanna buy it?