One reason, the studs sticking up around the selector makes it a pain. Especially the 4 long studs. I used some grade 5 allen head bolts I had on hand, kept meaning to switch to grade 8 but never got around to it...
Woop!! I got those ring and pinion gears installed in my truck a week or two ago.
YUGE difference... The 390, 4 speed and the 4.56 gears get along very nicely.