67's had lower compression pistons and closed chamber heads 68-71 had higher compression pistons with open chamber heads. So, at the end of the day the net results for final compression was about the same.
Adding gears, a higher stall speed converter, and a stout cam will make the single plane work better for sure, they just come on later than a dual plane, less low end, more top end so you have to accomodate that.
