Something you might want to consider is that if the mount kit you have access to will not work with the engine you have is buying a new mount kit with a Sanden compressor. The new compressors use much less power then the old York compressors that were used in the 70's and you would be starting out with a new compressor.

You will still be stuck with the front pulley wheel problem. You could always take the front pulley off the donor and see if it will work. If not then you are not out that much and you could be saving a lot.

And the A/C pulley is normally the one on the front and is not a part of the other pulley set. In the US A/C was often added at the dealer back then and with the front pulley being the one to drive the A/C you only had to add on a pulley, with the longer pulley bolt, and not pull the existing pulley set in order to drive your A/C.

Of course this set-up could be different. A/C was also installed at the factory if the dealer ordered it that way and their drive kits may be different. You will just have to look and see what you have when you get into it.

Also buy a new filter/dryer. The one you are getting might look great but it is always best to replace these if you are planning on keeping the car. I have used junkyard filters in the past when doing a similar 'install' on a car and sometimes they work and sometimes they don't. If you are planning on keeping the car for any length of time I would put a new filter/dryer in.