Factory cast exhaust manifolds or factory headers such as the shorties they put on small block Corvettes in the 80's were 1 1/4" to 1 3/8" I.D. tubes in general. 1 3/4" is more than your engine needs, especially if it is a crate motor or similar. What kind of cam is in it? Carb size and intake? Rear end gear ratio? It is important to tune all the parts to the application. The headers may be robbing you of a little low-end torque but the combination of parts will contribute to the issue.