slojoe; A few ideas:
1.) Make sure the connection tabs have the proper orientation. My connection tabs are pointing to the outside or towards the quarter window. The float assembly has a long reach into the tank and if someone previously pulled the assembly without proper re-installation, the float is against the side of the tank, holding it down.
2.) Check the wiring. Mine has the white/black forward, yellow rearward. The black, ground wire is under one of the small bolts. Scrape for proper grounding. Black ground runs all the way from sending unit to the common ground post on driver's side under dash.
3.) Since you were working with a new fuse panel, make sure your yellow/black illumination wire is not feeding the light of the fuel gauge.
4.) Lastly, if all else fails, as previously mentioned, pull the sending and and inspect for a float full of fuel holding it down or a broken wire.