Your best bet is to find someone wrecking an original HJ-HZ Statesman (or something else in these series) with factory air. Buy the whole firewall cut (cut trough windscreen pillars and across the floor under the firewall joint). You don't need instruments but see if you can get it complete. You also want the LH inner guard either to use, or to copy for where everything mounts. See if you can leave the inner guard hanging off the firewall attached by the A/C stuff, will have relays, the dryer, hoses etc on it. Grab also the condensor (in front of the radiator) and the mounting hardware. You'll probably have to replace this but worth getting so you have all the mounting stuff and the dimensions. Make sure you disconnect the vacuum feed hose off the inlet manifold and leave the inline non-return valve hanging off the vaccum hose hanging out of the firewall.
For a compressor you are better off trying to find an aftermarket bracket for whatever motor you are using that suits a Sanden SD508 compressor. Get the pulley, belt etc with it.
Then put it all in. Will mean totally stripping your car's dash, and moving all the A/C stuff across. There is a big 32mm hole to drill up in the centre of the firewall, there is a guide dimple there and the donor firewall will show you where it is. Same with the heater core holes, these are 3 x overlapping 16-20mm holes that you file into a slot, again guide dimples will show you where to drill. The A/C supply air vent that goes up into the plenum has 3 x screw holes (standard is 2), so drill teh 3rd hole and treat the bare steel before putting the duct in. A/C car has a blanking plate oon the driver's side but i'd leave the air duct there for fresh air supply.
Once it is all in get an auto air place to hook it all up. You'll need a new dryer, and i'd be getting a new compressor.
Also get the A/C heater core checked before you put it in.