Polyhead piston question?

Getting ready to rebuild a 66 318 polyhead for my 57 plym. I know alot of parts interchange with the LA engine, but what about the pistons? Just a stock rebuild with a mild cam. Pistons for the late model engine are a fraction of the cost? Any input would be helpful Thanks Chad

 

I think that the valve reliefs in the later 318 pistons won't line up with the valve placement in the early heads.

 

Pistons don't swap--totally different crown shape.

 

Thanks Guys! Chad