I cannot count the number of members here who has complained about having to downshift to 4th as you approach any kind of incline at highway speeds.
Any gear taller than 4.10s will render OD/5th useless.
2.5/AX5 with 4.10s is a 99% rule of thumb.
What went on with your donor? Who knows, as stated, after 33 years, anything. And there was a time when trucks could be ordered....again......who knows?
No one here gave you bad advice.