OK, I went out and did it over again, my L320 truck is positive ground, I didn't change it to see if it would work negative ground, too much work, I used 4 different senders(720/521/210-310/320) of which none were connected to the tank, I used 3 wires, one to the isolated post on the sender, one to the connection on the sensor housing, and one to a screw that mounts the sensor to the tank.
If I leave the wire on the screw that mounts the sender on the tank and switch the other 2 wires it shorts out the signal and the gauge doesn't move, it just pegs on empty.
So the results are that all the senders work(make the gauge move) when wired in the stock configuration, the 720, 521, and 210/310 all show full when the float is in the empty position and show empty when the float is in the full position, the two 320 senders I have both show properly, empty float position is empty, full float position is full.
I do not recall what happened a year ago when I tried using the 521 tank, maybe I just didn't want it showing full when it was empty, and since my 320 sender was good I used the 320 tank and the gauge works properly.
So just to see I went out and pulled the sender wires off the NL320 which is negative ground and got the exact same results as above, empty is full and full is empty when using anything other than a 320 sender.
Now I don't know if the fuel gauge can be re-wired or not, I suppose one could try(not me), I believe it would be easier to just restore the gauge face and reverse the "E" and "F".
So far I have not tried to get the stock water/coolant temp gauge to work on the NL320 wired as negative ground, Mike installed and wired in an aftermarket temp gauge and that works for me.