I would strongly advise against putting a hole in your tank to drain it. Spend the same money to buy any kind of cheap electric pump and a few feet of fuel hose. Remove the quick disconnect at the top of the tank [yes you can reach it from the bottom] and slip the 5/16 rubber hose onto the nipple that sticks out. No need to clamp it, you are going to suck fuel, not pressurize it.
Wire up a couple extension leads and alligator clamps to the pump, put the exit hose in a clean fuel container, connect the pump to a battery and pump your tank empty. Safely. Takes about 20 minutes to set it all up and you have no leaks after.
The empty tank will drop easily and install easily.
Try it.. you will be glad you did. All the fear of dropping a tank for repair will go away.
If you really can't get to the pickup tube at top of the tank, tap into the tank selector valve connection to pump it down. [be ready with a drain pan on your chest there, it will drain by gravity all over you while you make that connection]