Where oh where can a fellow find a fuel tank sending unit for an 84? I can find 85 and newer everywhere but no 84. OR. can I use a newer model sending unit in my 84? thanks guys

 

First, any sending unit you use needs to be for an '88 or older model, Ford changed the way the OHMs are used in '89, so your gauge won't work with '89 and up sending units.

empty-full
73-10 Ohms, pre-1989 Fords
16-158 Ohms, most '89-up Fords

So they are reversed

Good read here on rebuilding or replacing older sending unit with a universal type:
Fuel Tank Sending Unit Tech - FORDification.com

 

So.. Theoretically I could use one from an 86?

 

Yes