Well, here's what I did, and for the moment it is working. My car is a '91 that was originally R-12. When the compressor died 5 years ago, I ripped out all of the a/c parts with the exception of the a/c evaporator. To get the a/c going again I got parts from the following sources:
1995 200sx se-r (R-134a):
compressor and big metal line that goes from firewall to compressor. Free from friend.
1993 Sentra GXE in junkyard (R-12):
condenser, metal line that goes on top of the condenser, a/c evaporator. $50 for all.
1993 Sentra XE in junkyard (R-134a):
small line that goes from firewall to the metal line from the receiver/dryer. $5.00
Bought new: O-ring kit ($12) and receiver/dryer for 1991 ($12).
While I was gathering parts, I kept all open ends covered tight with plastic and in a dry, cool basement to prevent dirt and moisture from getting in. I'm not sure if it was necessary or not, but I didn't want to take the risk.
Lessons learned:
-Different compressors from SR20DEs may work as long as you have the bracket for it that mounts on the engine and the plug from that compressor in case you need to attach it to your B13 wire like I had to.
-A condenser from a B14 will not fit a B13.
-The R-12 condenser from B13 is bigger than that of a R-134a B14, but I don't know how the R-12 B13 condenser compares to a R-134a B13 condenser.
-The plug on a B14 compressor is bigger than the B13 plug, I just cut the B13 plug off and attached the B14 plug to the harness wire (as described in post #1 above). Works OK so far.
-The plug on a B14 receiver/dryer is too big for the B13 plug. I got a plug off a B13 receiver/dryer and swapped it over to my new receiver/dryer.
-Small metal line from firewall to the condenser's metal line on a B14 has two male ends. The B13 R-12 condenser needs a female end to attach to it and a male end to attach to the firewall.
-The boxes that house the B13 and B14 a/c evaporators look the same but bolt on differently, I don't think they can be swapped between models.
-It seems like the two female ends that stick out of the firewall into the engine bay from the a/c evaporator are universal in size between B13 and B14. However, the P10's looked different.
-The O-ring kit I bought was supposed to be specific to B13's, but in all reality I think the $4 universal kit from Pep Boys would have got the job done.
-Pull your a/c evaporator and clean it! All the ones I pulled were very dirty, including mine which looked like mice were having fun in the vicinity.
I can't think of anything else, but ask if you have any questions.