Since the Armando (or Aviaid) pans were not a factory item on any but competition Cobras, it's unlikely you will find an "original" dipstick will work. The factory dipsticks on most FEs came out of the block on the left front, near the
oil filter housing. You can use a stock dipstick in this location, but be sure to check clearance with the windage tray you likely have with the Armando pan. Finding one should be easy. Check Mustang, Cobra or Galaxie parts vendors on the Net.
If you want to use the nipple on your new pan, just go to your local auto parts store of choice and buy a generic chrome MR Gasket dipstick for a chevy big block. You'll also need a couple of small hose clamps and a 2" long or so piece of rubber hose. Carefully bend the lower portion of the dipstick so it lines up with the smaller nipple on the pan, running the dipstick tube up and behind the alternator. Use the top water pump bolt to attach the tube to the engine with the tab welded onto the dipstick tube. You may have to shorten the bottom of the dipstick tube an inch or two with a cutoff wheel or hack saw. Use the rubber hose and clamps to attach the bottom end of the dipstick tube to the nipple on the pan.
To mark the add/full lines on the dipstick, fill your engine with one less quart than full and mark a line on the dipstick. This is the add 1 qt line. Add the last quart and mark this line, too. This is the full line.
Should cost less than $20 for everything.