For sure, the 428 CJ was the best answer, but a "hot" 390 could have worked very well. The 428 CJ was a "Blue collar brawler" in that in was a relatively inexpensive option at around $360. Compared to an LS6 or Hemi or Ram Air II or Ram Air IV, it was a bargain.
But there was a lull of 6-7 years when Ford didn't have a good low cost high performance engine option. The 406's and 427's were rare and expensive. The 428 CJ was genius, in my opinion, but should have been there several years earlier. A true hi-po 390 would have been so easy. Of course hindsight is 20/20 and as you mentioned, Mustangs were selling like hotcakes anyway, so Ford's perspective and goals were not the same as mine.
JMO,
paulie