Help Installing fuel filler tube grommet and vent tube on a 69 Cuda fastback

I bought this 69 Cuda Fast back last year and am working out the bugs to make it drivable. I got all the electrical working now my next issue is the fuel filler tank grommet and vent tube. My 67 Cuda is a coupe so it was easy to remove the filler tube and Tank. This is a 69 fastback and it needs the fuel tank filler tube grommet replaced. The old one is just slid on the tube, it is not fitted on the tank. When I tried to fill it up, the gas came out the tank at the filler tube. The grommet is not fitted in the tank! just sitting outside the tank on the filler tube. The other problem is the Vent tube came with the box of left over parts LOL. So my problem is how to replace the fuel Tank grommet and install the Vent tube on a 1969 Cuda Fast Back? I though of dropping the tank and walking it out but I need to install the missing vent tube. What Interior parts do I need to remove to replace the filler tube grommet and install the missing the Vent tube? Any easy way to do this or does the back seat have to come out? Help