just splice the line before the fuel tank using a brass compression fitting with universal lines you would buy at an auto parts store, route it however you can by/along the frame past the fuel tank, and run it all the way to the rear axle hose. you probably need 2 61" long lengths 1 compression fitting, and 1 coupler, brake fluid, and a few cable ties. you will have to bleed both rear brake calipers. By dropping the tank, you could disturb fuel lines and wires for the fuel gauge, etc and it could turn into a much bigger deal than you think, especially since 1 line rusted out already.