It has been said several times over in this thread, but the connecting rod is the exact same length in all motors. The position of the con-rod on the crankshaft webs make for a longer stroke.
The 'D' piston skirt is shorter, but the position of the wristpin and top the piston is the same as the others. The port work is what differentiates the top ends. All can be ported for maximum potential, as well as pistons can be modified for maximum potential.
_The 70cc kit looks like it comes with a spacer that the 80cc does not._
Neither of these kits comes with any 'spacers'. A thicker gasket, but that is not enough to make the difference of hitting the piston at TDC on a 505/D.
We've had an 80cc kit going 55+ easy in Seattle on a 505/A, 505/D, 505/2B and 505/3B. We've had the 70cc going 45 with the stock carb upjetted on both a 504/1A and 505/1A,B&D.
You guys are really making this way too complicated.