Friend has a 90 f-150 that he is junking...would the rearend fit in my 78?

 

some Fords in the 70's had nine inch rears, most were danna, I think Ford went to a 8.8 rear in the early 80's If you need a rear diff I would stick with a nine inch, basicly indestructable, Chances are the drive shaft lenght's of the 90 Ford and your 78 will not be the same, check it out ..

 

I have seen this done before. You will have to reset the driveshaft angularity and shorten the D/S when going from a 9" to an 8.8" The rear u-joint may be a little different too. The 1979 @ the pinion yoke should be a Spicer# 5-1204X u-joint, which is 1330 series @ the D/S weld yoke and 1330, "special" at the 9" pinion yoke.