Gas guage Q's

[xfsketch]

72 chevy 350 4 blt main

Hey guys I have been experiencing some major gremlins in my electrical wiring all over this truck. The truck sat for a while & the PO unhooked everything. So far the oil pressure guage and the speedometer are all I have working. I pulled the dash cluster and sprayed the wire harness with some electrical cleaner and did the same for all the connections from front to rear, even on the firewall. I will post the other issues I have in a seperate topic. RIght now I need help with my gas guage. I read online that the stock tank on my ride is 20 gallons. Yesterday I put $35 in the truck right after it ran completely out of gas and the guage only read 1/4 a tank. YES gas prices are very high and I only put 11.5 gallons in it. My questions are as follows.

1. Is there a reserve amount of gas so when your on E you still have a few gallons left in the tank? Because I should have a half a tank but it only reads 1/4.

2. Could it be a ground issue? If so where is it located?

Any help is appreciated. Ive read most of the posts on gas guage probs and havent found one that has the same thing goin on.