Some great information above, this is why I love this forum.

The airbox / air flow meter (no mass air sensor in these old cars) is only necessary if you're going to use the ARV/EGR, many of us pull that out, you might not have the space anyway.

The A/C clutch is a great catch, as it has the speed sensor to turn it off if the compressor is seizing or otherwise has an RPM mismatch (slipping belt etc.). You could probably eliminate this circuit, especially if you're going to adapt your A/C compressor or eliminate it.

The 60x glow plug system is a parallel system. If it's a 3L 603 it doesn't have afterglow so it doesn't need a temperature sensor, if a 3.5L it will need the dual temperature sensor to run your aux. fan and the glowplug relay.

The vacuum system can be simplified, I believe that the BFS has been removed successfully.

Overboost protection is IMO hardly necessary with the stock settings and the stock turbo, if you modify the engine add a boost gauge. Without it you can also remove the SOV and associated plumbing, run the manifold pressure hose directly to the ALDA. The ALDA will help you keep overfueling to a minimum off-boost, IMO it is preferable to keep that.

MXFrank also mentioned the idle control plug, the round plug near the fusebox with numbers around it. This will be required if you want the idle speed control. Idle speed control is a nice refinement to compensate for the automatic transmission in drive and for air conditioning (and to a minor extent the alternator). If you are running a manual transmission and no A/C there will be very little effect and you could consider leaving that whole system out, and running the IP from the mechanical stop only.

The 603 alternator is internally regulated, should be easy there.

Pull the oil cooler of course.

You can go simple, and run all mechanical, or complicated to have all of the refinement of protections and a stable idle, depending on what you need.

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