I thought the 780 motor useds a 82mm polaris piston? and a 583 piston is 76mm with a 18mm wrist pin, simular to the 617. If this is the 670 your talking about and your only doing over bore and nothing to the rods you would be looking at about a 83.5mm piston, if you have enough wall left i would just bore to 85mm and make it an 800 as those pistons are much easier to get a hold of.
