Ok, lets see if I can address a couple of your questions...
First off your C4 will bolt right to the back of the 5.0L, however, you need to have the correct flexplate for that engine. They are balanced externally with a weighted flexplate and harmonic balancer. If you are talking a 5.0L, it is generally a 50 oz. setup. You older 302 was balanced with 28 oz. parts. So, if the donor vehicle where you got the 5.0L was an automatic, use thaTt flexplate. If it came with an AOD, your C4 torque converter should bolt to that flexplate. The Flexplate and torque converter are the only two things to address as far as bolting up your C4 to a 5.0L.
To address the Speed Density vs. Mass Air, the mass air setup is more flexible. The speed density can be a good setup, but it doesn't have the flexibility for performance changes such as a cam.
The computer that you should look for is out of a 5.0L mustang, however, there are a couple other cars that had the 5.0L HO engine, I know a few Thunderbirds and Crown Vics had them too. You can find a donor car, and get the harness and computer, or you can search ebay or other used car parts resources. The mustang computer you want is the 89-93, the other cars would be similar years.
A great resource for the EFI computer and needs is
www.fordfuelinjection.com
As to the gas mileage increase, it is there because of the nature of EFI, however most people are not switching for the gas mileage, the most common reason for moving to EFI is for offroad/rockcrawling, where the EFI is more reliable than a carbuerator in off camber situations. The mileage is a mere side benefit for these folks. I personally am moving to EFI because it is easier to run than a carb (most carbs are not properly tuned), with EFI, the computer does the tuning for you. If gas mileage is really your primary concern, you may be better off seeking an overdrive transmission at the same time to help with that....
I hope this helps a bit... if you need more help, just ask, there is a wealth of information on this subject out there!
What engine do you have? Where did you get it? Did the car you got it from have a computer and harness you have access to?
DD