Do those pistons have the big chamfer at the top around the OD?
It looks like they do in the picture, but they only have 4 valve reliefs in them. I looked at my pistons today and they also have the chamfer around the rim, but they have a huge dish in them too. These will have to have more compression, just by the difference in the dish vs. the reliefs.
Here's a link...
Flat top 307/250 piston 
