Well, lets see:
Dash lights: most likely the rheostat on the headlight switch is very dirty. Either clean it or replace your headlight switch. 71-3 Stangs have crappy dashlights anyway, so get used to that.
Horn: If you can remove the center pad, cross the two terminals on the steering wheel hub, if the horn operates, then you have a problem with the switch in the center pad.
Fan: check all your fuses, etc. The fan on Fords can be finicky, especially if the switch is old.
Short: Take your positive cable off, and put a test light in between it and the positive terminal fo the battery. If it lights, then you do indeed have a draw. Have someone watch the light as you pull one fuse at a time from the fuse box. If the light goes out, you have found the circuit the short is located on. If it doesn't, then it is located somewhere in the headlight or tail light circuit, as they are run off a circuit breaker.
Grille Lights: these lamp holders are notorious for corrosion, check them carefully, and that the grounds are intact on the radiator support. A test light would be very handy here as well.
Oil pressure gauge: check the harness and connections on the engine. This little harness is available new, and often suffers from age and abuse. Check continuity from the sender to your gauge.