It is very easy, I like the kits that Interactive Technologies or Granatelli sells. Easy instructions, simple to do with a few basic hand tools. Plus they come with a very nice harness, all you need to do is add your own meter, and computer and your ready to rock and roll!
I just looked it up at JEGS, WOW has that price went up! PT# 467-81868801 86-88 Mustang MAF Harness $79.99
Why do you want to upgrade if your motor is stock, the speed density system works fine in most cases then.