Depending on the mauufacturer, the 307's are total flattops, or sometimes they have a chamfer around the outside edge of the piston. They've got the classic V-8 valve reliefs in the top, but they don't have the dished center like the 250's have. I've found they raise the compression about 1/2 to 3/4 of a ratio, more if the head is milled at the same time. They're a direct swap, same weight, bore size, compression height, and wrist pin diameter. Just toss the 250 pistons, and install the 307's.
Jerry
