Mustang 5.0, spits, sputters, pops down the road. Any ideas?
Any engine codes? Smoke, knocking, pinging, etc?? If not, do a fuel pressure test. Get a fuel pressure gauge at your local auto parts store for about $20 dollars and see what the pressure is at. The fuel port is on the passenger side near the valve cover.
What did your spark plugs look like when you pulled them out? Were they covered in oil, fuel, or looked chalky white? This can tell you a lot.
I would clean the MAF sensor with a q-tip and alcohol. Pull the IAC (idle air control) and TB (throttle body) off and clean it up real good with some carb or brake cleaner. It could very well be your IAC. That can certainly cause idle issues like surging.
You can purchase a timing light for about 35-50 dollars and make sure your ignition timing is still set properly.
Also, you can get a compression tester for about $20 dollars as well and check each cylinder to ensure none are low. IF you do have one that is low then you might have a blown head gasket or cracked head which would explain the backfiring also.
I know it is a pain but it is simply a matter of troubleshooting and having the basic tools to do it. The good thing is the 5.0's just keep on running. They are strong as a bull. Go to a Mustang forum like moddedmustangs.com, stangnet.com, corrall.net etc. and post about your problem. Many of the guys there know so much about these cars. They will start simple with you. Check out one problem they mention. If that isn't it, move on to the next item and so forth. Eventually you will find the problem and hopefully it will not have costed you any more than some basic testing tools.