If you haven't done so, you need to remove the screws that hold your
filler neck in. This will let that side of the tank angle down so you can
turn, tug, twist, curse, yell or whatever to get the neck loose from
the rubber seal that's in the tank.
When you put it back in (get a new seal). Make sure you put some lube
on the I.D. of the seal and on the O.D. of the filler neck. What I did on my
'Cuda was this. I put the filler tube in the tank after I put the lube on and
let it set for a couple of days. I was thinking this might make it easier
to install in the car by "enlarging" the I.D. of the seal.
When I was ready to put the tank in I put a piece of 2 x 6 board between
the filler neck flange and the body. This let the neck stick a little
farther thru the floor seal and the filler neck wouldn't want to slide back up.
This method worked great for me...this was the 2nd time I had to do this...
my first new tank split on the bottom after about 2500 miles, but that's
another story.
Hope this helps.
Tony
P.S.
I forgot about the bleeding part...have some band aids ready.