I did something similar, a panel with the guts of the wiring slides up into the dash. held up with a couple spring clips
Be nice if it was a little larger to allow more room to lay out the wiring neater, but if I made it any bigger it would not have fit where I wanted it, which is on the dash side of the cowl vent to avoid any prolonged drips. As it is I can cut a few zip ties and get at every wire and every wire is labelled.
Putting a panel at the back of the cab where the factory tank used to live is tempting, but you are going to have a lot of wires running back and forth to the dash.
Making it slide is a bit of a pain, there is about a 1 inch bundle of wires in a service loop to allow that.
Hinged open would be easier to wire and for future access if you can find a place to do it.
56 3100 - rad fan circuit for me lives mainly on the firewall and fan shroud. A circuit breaker near battery, relay near fans, relay trigger gets power from ignition and starts the fans when the thermal switch in block goes to ground completing the trigger circuit.