On the fuel sending units, I would wire them up and check them with the key on before installing them. Just to double check they are working correctly, especially at empty. I would make note of what the dash gauge reads when the float is resting on the stop, that's when the gauge will stop reading and the fuel under that is your reserve.
Don't be surprised if the fuel readings have their quirks as you are using fuel from the tanks. By that I mean it might stay on full for a long time and then drop fast. Or it drop from full and then slow down in the middle of the gauge and then speed up again. This is just the nature of the beast, just so empty is accurate.
On the filling problem, what cured my filling problem was to take the roll-over valve out of the top of each tank, install a copper elbow that fit the rubber grommet, and then slip large rubber hose over the other end of the elbow and run it from the tank. I ran it up inbetween the bed and the outside body and tied it up and put a screen over the end to keep the bugs out. I did this for both tanks, make sure you use a tight elbow for the rear tank as it is smashed pretty close to the bottom of the bed. This has worked well for me, I can pump as fast as I want to, and even use the big truck nozzles, though I can't go wide open with one of them.
The only thing I have noticed is a little bit of foam will spit out of the hose when the tank fills up and cuts the station nozzle off.
