Yeah I got some of the 307 pistons also and used them on my last rebuild.
I'm thinking of checking the actual volume of the Depression on the 250 pistons and comparing that to the 4 valve cutouts on the 307 pistons. Alot of the 250 pistons on ebay have the big beveled edges and deep D shaped depressions on the top. these 250 ones I found have a very shallow depression. I dunno
