On my '84, i replaced the original axle with the later heavy duty one(from an '88). at that time i had the leafs rearched and an additional leaf added. I believe that the additional leaf was from a Ford F150 add-a-leaf kit, and turns out to be within 2mm's width of the originals. I also upgraded the rear shocks to the Monroe #555020 Thanks to the info that Lou(Trafic)) posted about them. I have had a few folks ask for recommendations on that rear shocks, and have found that the 555020 is no longer made. If you can find them, buy a set.
here is some additional tech info on the rear shocks;
for the MA815 & 550051; top stud is 3/8"-16, the bottom is a 5/8dia by 1-5/16" sleeveless rubber mount. they are 12 3/8" compressed and 19 7/8" fully extended. the max travel(usable) is 7 5/8". These are made for the 1990-2002 Ford Crown Victoria LX
for the 555020; top stud is 1/2"-13, the bottom is a sleeved rubber mounting, 1/2"id by 1-1/2" long. they are 13-1/2" compressed and 22" expanded. the max travel is 9". these fit the 1980-1991 Ford E350 Van
The MA815/555020 will probably work well for all highway driving, but you will want to watch out if you have to change a tire or otherwise force the shock to fully extended. a good pothole hit might destroy these shocks.
just more info for the group!