They are both tube full wave rectifiers but they are not wired the same. You need to change a couple of wires. I was looking at a supply this past weekend that used the 6z4 and the 12at7, 12B4A. I asked if it could be switched to the 6x4 and the said yes. I can send you the data sheets if you need them. One is in Chinese though! I have some NOS 6x4's so I was also interested in doing this. Let me know how it turns out.
