The transplanted engine would be difficult to verify as coming from a Mustang as a partial VIN was stamped on the block and may not incluced model type, plant or year. The range of years you have further complicates your quest. The VIN should be on the left rear block under the head (not sure, I don't have any cars or engines from that era). Federal law required the stamping but in typical fashion, federal law and the real world were (and are) far apart. The same applies to the transmission. You might want to do some reading in the FAQ section located in the Classic Mustang Technical Forum.