I bought one of those 3/8" fuel sending units for my 69 Charger. my 5/16 unit failed (the float sunk) and I figured since I had it out, I should upgrade it to the new one (didnt chuck out the old one yet). I filled up my tank and the gauge only shows 1/3 full....The float was new and didnt sink in water...the gauge is restored from AutoInstruments and doesnt even have 100 miles on the new cluster. I have soldered a ground wire to the fuel supply pipe and then grounded it directly to the chassis near the shock mount

What do I have to do to make that sending unit read correctly? Anyone have a schematic and a list of parts that I can solder in or wind up and make it work.....should I take the guts from my 5/16 sending unit and put it in the new one?

It's all in grounding the dash and the sending unit. A dash ground will usually mess with the temp gage too.