I've been involved with one rebuild of a 256, which was mostly used in WW II vintage Canadian military vehicles. It's an oddball combination of 248 and 270 components, and parts, particularly valves, were very difficult to find. 270s were used in thousands of WW II 6X6 trucks, and in the 1950's they were plentiful and cheap from surplus liquidators. Unfortunately, dirt track racers and Bonneville land speed record guys used most of them up. GMC engines respond to modifications MUCH better than Chevy. The survivors are scarce and expensive these days, and rebuild parts are getting almost impossible to find. I sent you a private message- - - -click on the flashing envelope icon at the top of the forum page. Good luck!
Jerry
