So it appears your glow plug circuit is operating. 15 to 20 seconds sounds pretty long to me, but I haven't owned an IDI in 6 years now. If it was me, I'd pull all the glowplugs, inspect and test them, but you could simply test them in place. Pretty simple process: remove the bullet connector and measure continuity with an ohmmeter, should be .2 to .5 ohms. I would remove them just to see what brand they are. Be careful removing them and if they are not Beru or Motorcraft replace them. Lots of history of Champions swelling and breaking off in the head.
I know your batteries are pretty new and test good, but have you taken a look at the battery cables and connections? See if you can get a good tachometer reading when you are cranking it cold. These are compression to ignition engines and if you can't spin one fast enough they just won't start cold. If your starter is original, I would be suspect as the compression in these engines are pretty ******* starters.
Again, gather a little more info and post your findings.