There has been some discussion about adding a fuel tank on the passengers
side in the space underneath the sliding door. The space measures 13" wide
X 54" long and looks like about 7" deep.

Would this tank work?

1971-1976 BRONCO 11 gallon front auxiliary tank WITH EMS fittings. Will fit
early 1977. Includes lock ring and O-ring. Dimensions 34"X13"X8 1/2"
Price: $149.95                  http://www.thejeep.com/mtstanks/bronco.htm

I guess the next question would be how to fill it, where would the filler go?

Gary