Here's a couple of guesses why nthe tranny was swapped. The original 4spd was probably a SM420. The 420 first gear was non-synchro and so low that you had to shift before you got to the other side o the intersection. Not too mention not real easy shifting though better than a 465. The passenger car 4spd is synchro in all four gears and lot easier shifting. Most folks with the heavy duty 4spd don't use first gear, unless they've changed the rear axle ratio. I've only found first gear usefull on very few occassions.
Larry

I don't own a vehicle that isn't old enough to drink.