Well the fuel gauge is wired with an ign feed then the other side goes to tank unit that should have an earth (brown) wire to ground. Sounds to me like the ign feed is interrupted when the lights are turned on? Check the light circuit and the fuel gauge is wired correctly.
I have never had a problem with bolt on covers, use cork gaskets . I also use clear Bostic on the gaskets to glue them to the covers. Well it works for me.
I fitted the early brake kit to my late drop spindles, needs an adaptor machining and the original calliper mountings removing. How to in Spliffy the panel build in gallery.
Just check the voltage (engine off) at the large terminal (battery +) on the alternator just to see if you have continuity. I drove all the way across Sweden and from the ferry to Malvern in my Dodge on the same fully charged battery from start. It was flat as the next morning!! The alternator...
Yes thats it just down the inside of chassis rail with grommeted holes at each end. Cant get to take pics at the moment unless there is one on the build thread in Gallery under Spliffy.