Thanks for the replies !
I'll check out all leads.
I know there are bound to be a few guys who have gathered up lots of the old 421SD stuff. I just don't know any of 'em.
Don't need a whole set. Just need one decent piston, suitable as a pattern for making a copy of it.
But, I suppose an official, detailed description MIGHT be sufficient to get NHRA approval. Or, must they see an actual original piston, to compare it to the copy ? Anybody know, for sure ?