Neither of those carburetor selections would be advantageous to making your thunderbird run better. First you need a good electronic distributor, and electronic coil, new plug wires, new spark plugs, and I would suggest a used Holley Street Dominator intake, used Edelbrock Streetmaster, or new or used Edelbrock Performer RPM, or similar four barrel aluminum intake manifold, a Summit 750 vacuum secondary carburetor, or Holley 3310 750 cfm carburetor and finally a good set of ceramic coated headers. I would also suggest a new roller timing set if the one in the vehicle is a few year old. Otherwise, I would not go into the engine until you see how well it runs with these modifications. If it needs a rebuild, then think about camshaft, compression, and possible stroker crankshaft kit at that time. Joe-JDC