This is for a 75 with the 4.9L
It currently has the Idiot lights, but I would like to have gauges.

What do I need to do?

 

Change the instrument cluster, main underdash wiring harness and the alternator wiring harness.

 

I sort of did this on my last truck, and mikeo0o0o0 is pretty much right. You would be better off to buy a junkyard harness, redo it to make sure it is good, and then just swap it in. I would recommend a later one as they have more fuses and circuits.

 

In all honesty, if you want accurate engine info, the factory gauges aren't worth the trouble.
The factory ammeters have been a POS from the word go. Ford even released a TSB on them because, basically, they don't work very well if at all, even when they were new.
On my project I'm going for originality. No, my ammeter didn't work. I located a NOS ammeter and spent over a year looking for a NOS alternator harness and finally found one.
Now, all I can do is wait patiently to finish my project and "hope" my ammeter will actually work.
Aftermarket gauges are much more accurate and much easier to install and will actually work.

 

As I recall, you also need to swa the sending unit in the engine for the guage vs. the light.

 

Correct, the oil pressure sending unit needs to be changed.