What kind of modifications are you considering?? There are simple things like a solid differential location kit that replaces the bushings with solid aluminum, spherical inserts in the rear control arms, an adjustable toe modification (uses a bolt for the adjustment, not shims) for the rear controls arms, an HD rear differential cover, heim jointed toe rods, heim jointed camber rods... etc etc... all the way to a 5 bar rear suspension... along with various spring rates and shock valving.
The front end has the typical modifications available... control arms, del-a-lum control arm bushings, spreader bar, heim jointed stay bar (sway/stabilizer bar) end links, various spring rates and shock valving... etc etc.
Gearing available:
2.73
3.08
3.23 OEM (no longer available, except good used)
3.36
3.55
3.70/3.73
3.90
4.11
4.33
4.56
There are even 12bolt internal conversions.
A couple of links to some stuff...
http://www.tomsdifferentials.com/catalog.asp?pg=17 and
http://www.guldstrand.com/scripts/prodList.aspI used to work for ole Dickiepoo Guldstrand building F and Y bodies. All of his stuff can be DYI as it is pretty straight forward. Not to mention that it has been more than field tested per-say.