I am no expert on casting numbers, but I do know the Boss 302 and the Cleveland had similar head castings. It's quite possibly a Cleveland. You can tell a Cleveland from a Windsor by where the radiator hose connects to the engine. The Windsor goes into the intake manifold and the Cleveland does not. The Cleveland also uses 14 mm plugs vs. the 5/8" plugs in the Windsor. Also, the D1 would indicate a 1971 casting vs. 1969 so it is not the original block that came with the car. Poke around in this site, there's all kinds of Mustang info in here.
http://links.vintage-mustang.com/index.html