Simple explanation is :
Rangers built with Calif emissions have the EGR tube, O2 sensor port, and the hot air tube so they are a direct bolt up to the mustang. Otherwise some have the o2 sensor port and some don't. If you get one that doesn't you'll have to have a bung welded in.
The more detailed explanation is that 91-93 8 plug 2.3, no mods needed but I believe 87-90 2.3 motors seem to need the tube modified since their upper intake manifold is longer and places the egr valve a little higher up.
So if your one of the 87-90 guys, egr valve will accept the ranger's egr tube, but the ranger header will not accept the mustang egr tube, so make sure you get the egr tube with the header if you're planning on keeping the egr and you'll have to use both to make one that works or delete egr on the mustang. I went the delete route and used I believe a 1/2 pipe plug to get the job done.
I used a mustang exhaust manifold gasket, with the bolts from the Ranger because the Rangers bolts were 2 or 3 threads longer than the mustangs stock bolts.
You should be able to pickup a header with the egr and bolts cheap in any junkyard.. I did this swap maybe seven years ago and at the time I paid I want to say $30 at the yard for it all and something else for another car lol so in my case the added low end torque was worth it.
Another cool thing about it is that the stock mustang manifold ways 26lbs verses the Ranger header which was 6lbs.. So shaved a few pounds off the front of the car and well the aluminum driveshaft I had from another car and decided to toss it in because it was a 5spd so it fine. It don't know if it made a difference in umph but with the header and ds I seen a good increase in mpg with the turd
Hope that helps some