1966 C-10 Short Step

I just moved my gas tank from behind the seat to the rear of the truck. That went well. I installed the sending unit and grounded the unit to the frame. I ran a new wire from the new sender to the fuse panel and plugged in the wire to the panel. Worked good. I added a few gallons of fuel to make sure the sender worked properly before I reinstalled the bed wood. I moved the truck to the opposite side of the garage and the gauge went to full and stayed there. Tank is approximately half full. I went to the fuse panel and noted that the bottom screw to attach it to the firewall was missing. I inserted a new screw and as I tightened I the gauge worked again. The went to full again and has stayed there - almost like the whole system is grounded.

My question is - is there some form of insulator that goes under the fuse panel - like a spacer or something?