[flatheadmurre]

Round pistons will be non round when at work temperature...
That they are tapered and oval shaped is for compensating that they expand more at top and that you have a pistonpin in them affecting expansion.
As long as you keep them tapered...straigt they will either seize at top or make a heck of a noise being loose at the skirt...
Measure a finished piston that will give you the ratio between top/skirt...that will give you a starting point...