As this is a mild street applicaiton and drivability will be a concern, building a 318, or even a 360, would be best. The 273 is nice and it is original, but it lacks torque and the small diameter bore has impacts on air flow into the cylinder. This limits HP to some extent. I built my 340 in 1974 with a 318 cam and a small manifold as the energy crisis then made fuel economy paramount. This combo with a 4-row radiator and electric cooling fan ran 14.50's @95 and got 25-26 MPG and was able to drive in Tucson in 117°F heat and not run over 190°F. Ideal combo that can be duplicated with a 318 or 360….