One way I've solved this issue in the past is to run a wire of sorts through the firewall, and into the fuse. I've used wire coat hangers many times for this. Then, slide the fuel tubing over the coat hangers, and pull the coat hangers out as you push the tank in. The tubing will come right out the hole with the coat hanger, and the tank should slide right into place for you. Just remember to run two wires up into the fuse at the same time, so that you can pull both fuel tubes through at the same time. You might have to get something like kitchen tongs or the sort to help if the tank needs to go up into the front of the fuse farther than your hands will push it. If necessary, consider cutting a tank hatch in the bottom of the fuse for access to the tank compartment.