225 0r 175 slant six

On the practical side, the heavier the car your putting it in the more appropriate the 225 becomes. In the early "A"'s I drive ('60-'62 just over 2800#'s) the 170 does very well for all the reasons other posters mentioned. If your body is a heavier '75 Duster with air, power brakes and steering, the 225 is probably the better choice for a typical city driving cycle.