The best way to identify a particular block is with the casting number, down near the oil pan rail on the passenger's side front. There may (or may not) be a stamped, not cast letter/number sequence on a machined pad immediately behind the distributor. Original equipment engines had the stamped number, linking the engine to the vehicle in which it left the factory, and its size. "service replacement" blocks were supposed to be stamped by the installing dealer, but many weren't. The only surefire way to identify a 261 is to pull the cylinder head and measure the bore. (3.75"). 235 and 261 engines use the same crankshaft- - - -3 15/16 inch stroke.
Jerry
