Paul,
I changed my injectors 2 years ago in my 88 XJ for safety. I had read multiple posts about the Benidx injectors leaking between the metal body and the plastic portion of the Renix 4.0 (87-90).
I have the EFI manual from AMC/JEEP for the engine. It talks about have to replace the quick connect coupling when you disconnect the line from the fuel rail. It is those plastic ears sticking out. You squeeze the ears then pull apart. Then use a paper clip to pick out the 2 o-rings and the spacer. Then install a new "coupling renewal kit". You get the kit at the dealer and they run around $15. I used 2 kits when I changed my injectors.
I was not bold enough to change to a different injector so I searched the on-line web sites and found the best price for stock replacement injectors was at
www.rockauto.com. I installed 6 "Standard" brand injectors. If you want to go with the mustang 5.0 injectors why not contact one of these injector rebuilders, tell them what you plan on doing. They may bring up something. A guy at work has a relative who does rebuilds. The relative was telling the guy that not all injectors are not interchangable due to the resistance of the coil. I have read the injector swap lots of times on here but I would check out the full interchangability before you do install alternate injectors.
The EFI manual said you should get 13 O-rings in the kit from the dealer, 12 are used on the injectors, the 13th is used on the pressure regulator. You remove the two screws attaching the regulator, pull out the regulator, change O-ring, and re install.
With the age of my XJ I decided to spend the $65 and buy a new regulator since it was just those two screws that kept it on after the fuel rail was removed.
I used motor oil on my o-rings but my dealer mechanic said later petrolium Jelly, Vasoline is a brand name of it, would work just fine. He said about anything to give a little lube as you slide it in.
I found removing the clip on the electrical connector make removal easier, I am talking the clip that is about the same size wire as a paper clip. Once you have the connector off, reinstall the clip. Then when you reconnect the harness to the new injector it will just "click" on.
Also mark you electrical wires so you do not hook them back up in the wrong order. I just used colored electrical tape and marked the wire the connected to the more "forward" injector of the two. You will notice those pigtails come out of the harness is pairs.
You will need to bleed down the fuel pressure before you start. Just press the schrader valve on the side of the fuel rail. Also if you can work on a cold engine, one that sat overnight just to prevent any chance of fire.
Hope that helps, I am sure I left something out.