blocking off fuel lines

[Toe_cutter]

The first of August I traded my daily driver ( Grand Marquis) for a truck and more cash than I had in the car, its a 95 F150, 4.9 efi, m5odr2 , 3.08 rear. I decided to start a good fuel log and keep track of every gallon I put in and mile I drive. I got the truck from a buddy at work and he warned me that the front tank (twin tanks, 22ish front and 14ish rear, the lines just Y off, no switch valve) did not work. A new pump is almost $400 so that's out of the question, so I just sucked the gas from the front tank the first weekend I purchased it, with aquarium tube. Four tanks later I get home from work and find gas leaking from the front tank filler tube, check valve is leaking in the return line on the front tank that is non functional, causing cross filling. Sucked just under 22 gallons out of the front tank, crimped the return and pressure lines shut with vice grips but they are not holding 100% as I have pulled another 7gal out in the last two weeks.
I have just about run through the 22 gallons I have sucked out the front and added to the rear, and put my mpg tracking on hold for the last few weeks, but now I have to find a way to completely leak free block off the lines and I guess ditch the tank.
Fuel pressure on the 4.9's runs at 60psi instead of the normal 40psi of most vehicles so im kinda scared to just jam a bolt in there and hose clamp them. like I said both the supply and return lines are just Y'd off and there is no switch valve, switching is done by internal check valves in the pumps. The lines are all rubber and there is not a replacement part number for just an aft tank that I can locate. what would the best way to block them off be???