My 73 has the dealer installed AC. Like many, it doesn't blow much cold air, and I've confirmed that it does have a full charge of refrigerant. I'm getting ready to swap in a 280 motor from an '80 280zx and I received it's compressor in the deal.

My question is if the new compressor is compatible with my existing system and if it would improve its performance.

Given I have the engine out, there will never be an easier time to make the swap!

In a word YES.

Even if your old system had been upgraded to 134a the new compressor will still work find. That is, if it's a good compressor.

I just did a complete overhaul of my ac system and removed the dealer air and factory heater.

I replaced it with a Souther Rods system.

Hoses, bulkheads for the fire wall, air handler, and condenser.

Before hooking erverything up I had the compressor checked and found that it was not working at full capacity. Since I was going to use 134a the compressor needed to be replaced.

So just have the compressor checked out first.

Good luck.