What fuel cell fits nicely in the spare tire well 70 Dart?

You're right!
After doing a little math to fit a square peg in a round hole, RCI makes a 12 gal cell 17x17x11 that drops right in.

Now, because of the sump hanging down, do I need to make a support for the back half of the cell to prevent sagging?
No i wouldnt think so your not cutting that much of the car to worry about it the bottom of the tank will sit on the well floor an the sump just hangs out in the hole.i made my first 12 ga tank sit on the floor an built the sump hole around it an not taking any of the weight,so it was just hanging there with no weight on the sump only the bottom of the tank(u get it?) i would put down some double sided pading so the tank wont vibrate under the straps an drive u nuts,an to prevent long haul wearing of the bottom!u get what im trying to say?