How much taper wear do the cylinders have? If there's less than .005" of taper, a re-ring job at .040" oversize won't be a problem. I've built a few 250's to .060" oversize, but they were short-life race engines so a long-lasting rebuild was not a concern. The 307 pistons have all the same dimensions as the 250, including weight, so they're a direct swap. They just have flat tops instead of dished, which raises the compression. I've also done a .050" head mill job on a few engines along with the 307 pistons without getting into piston/valve interference problems with a stock-lift cam, and the engines still ran regular gas without detonation problems. The 292 exhaust manifold is a good suggestion, but they're a little hard to find. If you decide to find another block, the 230 can be used to build up a 250, with just a crankshaft change. The bore size is the same.
Jerry
