Was your adjuster the type with the long slot (ETC6291) - infinitely adjustable but can self adjust (!) which is why the later bolt one is better.Seems its a similar issue that I had on my 3.5, I had to remove the bolt on the belt adjuster, loop the belt over the two pulleys and use a crow bar to leaver the alternator to to stretch the belt and fit the bolt back into the adjuster.
Using a bit of logic before Forums.
Yes it was the long slot version, but still the same issue the problem is the high pulley shoulders, I never saw the bolt style of adjustment until the 3.9 came out.Was your adjuster the type with the long slot (ETC6291) - infinitely adjustable but can self adjust (!) which is why the later bolt one is better.