I need to change the float so the sending unit will work. With a 3" body lift on my '79 F-150 4x4, it looks like I have enough room to do the job without removing the tank from the truck. Is it absolutely necessary to remove the tank in this case?

 

So long as you can get to it, it's not necessary to remove the tank. Most of them have a ring with ears on it, that you take and tap around to release the unit.