Auto Trans Interchange?

["Dave C." <conner@cfm.ohio-state.edu>]

Due to the worsening leaks in both my cars' (87 5KS and early 89 100, both
with NF engine) auto trans', I'm exploring my options. 
I've located a potential donor car, an 85 5KS with apparently rebuilt trans
showing zero signs of any leaks.  So I'm considering buying it to swap into
one of the cars.

What are my options?  
  - Will the 85's tranny work in either, or both, of my cars?
  - Should I swap it as a unit... including trans, diff, and torque converter?
  - What is the significance of the different final drive used in the 85?
  - Should I replace the oil cooler with a new one?


All three transmissions are type 087, but there are differences.
Here is what I know about each...trans code and build date copied off the
trans, the rest of the info from Bentley...

85 5KS (Donor)...
  Trans Code = RDN
  Build date code = 07043X   (I think the X means rebuilt?  Would that be a
swap of tranny only, or would it also include diff and torque converter?)
  Torque Converter Code = G
  Final Drive ratio = 3.45

87 5KS
  Trans Code = RDX
  Build Date Code = 09037
  Torque Converter Code = Q
  Final Drive ratio = 3.25

89 100
  Trans Code = REH
  Build Date Code = 30068
  Torque Converter Code = Q
  Final Drive ratio = 3.25

Bentley show a different oil cooler on each of the three.

FWIW, I asked at the local salvage yard about the interchange, but doubt
what they told me.    
They said the trans from the 85 will work in the 87, but not in the 89.  
  Does anyone know if this is true?
  If it is true...does anyone know >why< not in the 89? 

I've never done any transmission work, but I am comfortable with the
removal and re-install.  
I'm also willing to try my hand at replacing seals.  My >hope< is that I
could install the trans from the donor car into one of mine.  Then reseal
the one I removed and eventually swap it into my other car.  I don't know
what I'll do with the extra (leaky) trans when it's all done...keep it as a
spare maybe.  Or maybe I'll go into the Audi transmission resealing
business.  Seems like there is a demand for it.  Some listers tell me it
isn't too difficult, but I'm not so sure.  It seems that in addition to
replacing seals the disassembly would require a lot of adjustments to be
made and tolerances to be checked.  Anybody BTDT? 

Dave C.