Update #2
First off, I want to say thank you to VetteVet for the link to 6066 GMC guy, lots of wiring info. I tested my gauge by ground the sending unit wire and it does in fact go to empty so I am confident that it is working. I then monitored the ohm resistance as I filled my tank on sunday ( used my multimeter from ground to the sending unit wire ) and the resistance went from 31 ohms to 104 ohms nice and smooth as the tank filled. Soooo, I now know that the sending unit is working as it should. I removed the 45 ohm resistor from the back of the fuel gauge ( only two posts on the back of the gauge, one for the sending unit wire and one for the keyed 12V power ) and checked the gauges resistance - its 47 ohms.
Hooked up the gauge without the 45 ohm resitor and now my gauge pegs at the empty mark!
At this point I pulled out half of my hair, started speaking in toungues, made obscene gestures towards my truck and called it a day! Am I beating a dead horse trying to get my factory 30 ohm fuel gauge to work with this newer (1978) 90 ohm sending unit? Is there anything else I can try to get this pieces to work or do I need to consider other avenues?
As always, any and all help/info greatly appreciated.
Pete