I have used the Harbor Freight glue puller on my 78 Suburban which is the same thickness as your Ford with much better success. The glue that you are using is also a low temperature glue and will not stick and hold as well as other glues. I also learned that through trial and error. I suggest buying the kit from ebay and trying again. You should also buy a soft mallet which you can use to help straighten out the high spots which will help straiten the dent out while pulling with the glue.
It was pretty stuck on there and it wasn't moving. I belted it out with a rubber mallet from the other side. I don't think a puller would cut it. I should get one and find out!