I recently purchased a 1965 Mustang Fastback, older restoration in very good shape. The car appears to be all matching numbers or original with one exception, the steering box has been upgraded to an early 67 performance unit(quicker)?. The engine is in need of a rebuild and I have a fresh 67 289 engine. My question is, should I rebuild the 65 engine or use the 67 engine? How much value would be lost with a non matching numbers engine? This car could be turned into a very good number one with a small amount of effort. Is this the only place where any body cares about numbers matching? I don't want to lower the value of the car to save a few bucks on a rebuild, on the other hand I have an excellent engine in the corner begging to be used. Thanks for your input, teradactl