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I have a 260# 2 stroke motor on my classic 15' SS with a mechanical jackplate. The boat doesn't sit that low with the jack plate (the water line is at the rear splash well). The 70 Mercury of the era were 275#. I think if you used a new Yamaha 70hp (which weights about 260#) with a light jack plate that you would be fine. Any other current 70 or 75 motor is going to be too heavy. I am pretty sure the 13' and a new four stroke 40 is going to be too heavy as well.

IMO the jack plate is worth 2mph. I have my motor raises another 2.5" higher than I could have run it with the motor on the transom. It planes off easily (much better in fact than the too low of position that it was mounted when I purchased the motor). If you into speed (or higher cruise speeds). The 15' with 70 is almost 10 mph faster than the 13' 40hp, plus it has a much higher cruising speed. The 15' ride better then only down side besides cost is the 13' has more lateral stability and a slower planning speed.