The 250 piston has a depression in the top that's almost 1/4" deep, and about 2" in diameter. The edge of the 250 piston also has a 45 degree chamfer. Most of the 307 pistons are flat top. The last 250 I built had a .030" overbore, 307 flattops, and .050" milled off the head. It still runs well on regular gas, with no detonation. I was probably on the ragged edge of needing to go to mid-grade gas, but the engine has been in service for several years with no problems. I built two others with 307 pistons, but did not mill the heads. The milling job adds a little low-end torque, but they all ran about the same at cruise speed.
Jerry
