What camshaft should I use?

10mpg thats no good my 318 gets 15mpg with 3.55s i would check your base timing first and limit the initial with FBO limiter plate and mr.gasket spring kit Slant six like timing.

Once you figure all that out ,on your camshaft i would go with this nice little bump up High torque low Rpm Power Band Howards Cams Mechanical Flat Tappet Camshafts 751082-12