I just finished building a 261 and elected to not run a filter and just change the oil every couple of thousahd miles. Drilling the head is pretty simple. I just tacked the head gasket to the head with a little silicone, marked the holes between the combustion chambers and drille into the water jacket. Performance wise you'll get a small horespower increase (approximately 10% per the manual). The torque increase is only about 4% per the manual; however, the torque curve is flatter and of longer duration. I did the Offy intake, 2 Carter/Webbers, mild cam and Fenton headers with dual exhaust. Difference over the stock 235 is daylight and dark.
