Sounds like you have the same luck as me with cars lately.
1) Are the pulleys all properly aligned now? Is there a ton of belt dust around the blower pulley area? I have a laser alignment tool if you need some help checking if all the pulleys are lined up properly.
2) Not sure about the cluster swap since yours is older. On mine it was a direct swap since it's a '00 and I used the '03 Cobra wiring.
3) Always reading full could be a problem with the sender, wiring, or gauge. If you have the '03 tank and sender then swapping in the newer Cobra cluster might even fix it. I would disconnect the sender at the tank to start troubleshooting. If it's like on earlier Mustangs, it should read full when disconnected and grounding the pin on the connector from the sender should make it read empty.
Bro, I completely forgot you had already done all of this. I really need to show you the car, sit down and pick your brain with this car. I need to get the bugs worked out of it so I can go forward with it. Got some fun ideas in store for this car. :rockon: