Jim:
First,....1.75" headers are too big for a 3.0 engine unless its a very aggressive, 8500 RPM, high HP (360+) motor.
Second,.....1 5/8's is a MUCH better choice; 1.5" ones will work depending on camshaft profiles.
Third,......Carburetor choice depends on camshaft profiles and how many RPM it will run at.
Then,.....Jetting is based on displacement, CR, cams, intake manifolds, and exhaust system.
One really cannot make useful recommendations without all that information finalized.