Before you remove the tank, get a new tank strap, as this will almost defo be rusted very badly, there is only 1 strap, uses a twist and lock affair at one end(rear) of car, the other end bolts to a captive nut, as do another two bolts that go through the flange of the tank.
These captive nuts are on a small metal bracket that can still be got from ford, so when the old ones rusted together so bad they break, just grind off use long nose pliers to fish old bits out and use new captive nuts to secure.
New straps can be got in polished SS from Auto Specialists, advertise in FF etc, or a panel supplier like hadrians panels.
You will also need a new float seal, there are two sizes the cossie uses the small size, but the part numbers are a confusing for these, together they only cost about £3-£4 so get both and take back wrong one.
The actual filler seal is normally perfectly ok to re use, some pattern tanks need the main breather from tank lengthening by about 1", this is a moulded part on the filler neck but has a loose section so can be cut and use some opa pipe or similar, make sure it is a nice tight secure fit if you need to do this, as otherwise filling can result in fuel pissing out the breather hole down the breather tube past the added bit and all over the floor.
Remember it is 100% vital to note where the two rubber pads sit between the tank and body/chassis areas and these need to be glued back on new tank, otherwise it will drum like buggery when not full.
You'll never get all the fuel out, I just remove the tank to pump pipe and rain that way into jerry cans etc, then when stopped flowing just lower, they are not that heavy can easily be done with one person, I never needed to remove my exhaust at all, just removed the rear bumper(4 bolts inside boot).
tabetha