I have built several flattop piston 250's using 307 pistons. They're exactly the same, weight, pin diameter, and compression height. The dish top/flat top is the only difference. Since the pin is a press fit in the rod, there's no small-end bushing to be concerned about. I've combined the flattops with an .050" mill job on the head to bump the compression a little more, and still ran 87 octane gas.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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