why do you have to leave the compressor on? You should be able to get the correct alternator mount and run just the alternator, then you would have that much more room. On my 84, I bought an alternator bracket kit from jc whitney for like 20 bucks and got some spacers (I currently have a 14 mm socket between the rear of the alternator and the head) It spaces the alternator out some, so you can only run one belt, on the outermost groove for the alternator, to the inside groove on the alternator. That should give clearance for the water pump castings.
If you do decide to leave the compressor on, just disconnect the wiring harness at the alternator so that it cant accidentaly be turned on, which im sure would make a terrible mess.
C.J.