I like the passion for seeing Christ in the old testament, but this misses the mark. There is not enough Bible evidence to point to theophany. Rather, Melchy is a type of Christ, not Christ himself.
Melchizedek was a theophany: an Old Testament human manifestation of the Second Person of the Trinity. Christ fulfilled His own theophany by becoming incarnate, uniting to a human nature. He is our High Priest forever.