Yes, it will fit, but use a '69 sending unit or your guage will be off. You will sacrafice a little trunk space to accomodate the other 2 gallons ('69 is only 18 gallons, it went to the 20 gallon in '70). The bottom of the tank will be in exactly the same place as the old one, the added capacity is in the height of the tank inside the trunk.
Mustang Monthly did an article a few issues ago on that very topic. They showed how to put it in step by step. I can't remeber exactly what issue it was though. I guess it works pretty well and they highly recomend it.