Autolite 3924.  Gap based on the ignition system.  In a typical TFI EFI engine I would start at 0.045", and could go as high as 0.060" depending on what coil you are running.

Good to hear that you are still hammering away on it, Graham.  The Fox in '83 was not injected...and the 5.0 designation came out with the change to roller cam in '85.  Another thing...if you are using an 83 engine I'm not sure about the firing order as I don't know when they went to the 137xxxxx FO from the older 154xxxxx order.  If you are going with the Ford EEC-IV you'll need to wire the dizzy for the cam FO but leave the injectors wired to run the 137XXXXX FO.  Otherwise the the HEGO's will be sending bad into the the PCM and things will go downhill from there. 
From what I've read having this FO miss match may cause a slightly rough idle but will not affect normal driving.

BB

Thanks for your help so far and Hi to you Bob.
The engine came out of the Fox body Mustang running beautifully but I had to swap the sump and decided to strip the engine to check it out. It looks brand new inside. I will check the casting numbers on the heads to see if I can get a rough idea of the engine date. There is a label on the valve cover. I will try to get a photo of it on here to see if it tells anything too.

I Have looked on the valve cover of the engine and there is not much writing left.
there is a big AL078623 505
another code ending in 36AA
and a date 06/19/91.
I can definitely make out the AL, the 36AA, the 505 and the date. The other numbers are to the best that I can see/guess as they are very faint.
Does this help any?

Took a look on the heads and there is a cast in plate with 1F 6 on one head and 1F 7 on the other.
That doesn't sound right does it?

Fantastic! Thanks very much. So which spark plugs would you recommend for this? it is only normal use in my 66 Mustang.

Unfortunately that's not the number you need to tell you which plugs to run.  With the engine of unknown origin you can't say for absolute certain that just because the heads were cast during the E7TE production run that they are definitely E7TE heads.  You have an EFI engine from an '83 Fox Mustang; a car that never had EFI from the factory.  Maybe the engine is from a later year Mustang, but its just as likely that its from a '91 truck, van, Crown Vic, etc.  Some of those other vehicles used E6SE heads up to '91, and those two heads do not use the same plugs. 

It occurs to me that the casting number is actually on the bottom of the head on the exhaust side, below one of the two middle ports.  I don't know if you can see it with the heads installed, but maybe with a mirror, etc.