Temp & fuel gauges - what am I overlooking?

You know all that yet you can't get it to work. We have been beating our heads trying to give you advice, and you don't directly address this. I don't know where your are or where you've been. Worse, I don't know where you are going. It seems to me you are chasing your tail. This is the last I'm going to say

I already told you to wire up the cluster on a bench, use the test resistors, and get the cluster/ gauges to work on resistors. If that works then you have gotten that far. If it does not check out, you need to address it before you even think about the rest of the vehicle

The sender wires are simple, and there are not that many terminal points

The fuel sender wire has t he terminal at the cluster connector / pc board so THAT is an issue. It has the left kick panel connector and that can be an issue. The sender wire can get pinched where the back seat goes. The sender connector can be corroded/ damaged, and the sender can have several problems

The temp sender wire has the same type cluster /pc board connector, then the terminal at the bulkhead / firewall connector and the same type sender connector as the tank

AFTER YOU get the CLUSTER tested on resistors, you should be able to install it, and then retest using resistors AT THE SENDER connectors

Once that checks out, the only thing left is senders themselves, and possibly the sender grounding. This can be a real issue with fuel tanks, and the temp sender is easily damaged by over-tightening.