Gas gauge/tank problem ('74 C20)

[MWS1994]

I have a 1974 C20 and the gas gauge has acted weird since I got it a couple months ago. It's not road legal so I have just started it up to tinker with it and drive it around the house. It has run out of gas a couple times and I only put in about a gal at a time.

Now for the weird part. I only put gas in the left (drivers side) tank but the truck will only run with the switch flipped to the RH side (that does mean the right side, right?). When I switch it to the LH side, it runs for a little then sputters and dies and the needle goes to the full position and then slowly goes to about 3/4 full. When I switch it back to the RH side, it starts up and the needle sometimes stays where it was (3/4) but most of the time it either read 1/4 tank or fully empty. I have searched around but couldn't find any answers.

Anyone have any ideas?