New forklift tanks are designed to function and be stored for transport in either the upright or horizontal (index hole in the down position) so the pressure relief valve is in contact with the airspace in the tank and the fuel delivery tube gets to the bottom of the liquid.
I have crossed the scales many times with 4 forklifts with tanks on them and DOT never said anything about the tanks.
As far as fuel transporting, our company policy is, (6 tanks or less in a cage = no hazmat placards or endorsement needed) over that we have a ventilated trailer with placards etc. It doesn't matter if the tanks are empty or full.
When I worked in Manhattan any fuel delivery had to be by a special contractor with tunnel permits.