Thinking of getting a slant 6 dart.

A slant six w/ 1 bbl is not big on horsepower but fine on torque, which is what the public terms "power". They were put in many trucks and vans, so have no trouble moving a light A-body. To realize max horsepower in any engine, you must scream it at 6000 rpm. Few people drive around like that, except dumb kids in little Jap cars. In my experience, there are many poorly rebuilt Holley 1 bbl carbs, especially the 1920, so you might spend time finding a good one or getting someone competent to rebuild it.