I also put a Spectra efi tank on my 74 Challenger. Before I installed the tank, I made a half hearted attempt to remove the sending unit / pump assembly. I didn't readily see a way to get it out unless I drilled out a couple of rivets so I just sealed it back up and put it in the car.
It seems to have roughly ten gallons in it when the gauge reads one quarter.
I don't know how different the Coronet tank is from the Challenger tank, but I would like to hear how you removed yours.
I think it may be easier to reassemble a 1911 or AR platform blindfolded when compared to getting the pickup unit out of the tank....and back in without mangling it all up. It was about an hour of trial and error in getting the correct angle and "push/pull" on pump base. The biggest hang up is the rubber nipples that protrude from the isolation seat/bracket. Those little things will hold it all up from coming out. The locating tang cutouts are your friend on the tank and that is where you have to line up the arm and other larger pieces. It comes out at a position of 90* or more when you figure out the combo. The plastic float WILL come off and so be ready to fish it out and PRAY that the small lock washer that keep the fuel sock on does not come off during the removal.
