8 3/4" from a 1/2 Ton P/U or Van For "A" Body

My family has had 4 69-71 pickups.
All 741 case, 3.91 open gears, except one my dads 70 he bought new it had an optional 3.55 rear??????????
All four trucks were stick shift.
Dad said grandpa's long wheel base with 3.91 would get better milage than his short wheel base step side by 2 mpg.
Same motors 225 3 on the tree.
Maybe the step side be had something to do with areodynamics.