Your stock fuel injectors (1986-88 non-HO 5.0) are rated at 14 lb/hr and are light grey in color. The HO computer is set up for 19 lb/hr (orange or tan) injectors. You'll absolutely need to acquire 19 lb/hr injectors for this conversion; you can use them from a Mustang or a Mark VII. The good news is, they're fairly plentiful and usually very cheap. Just ask any Mustang guy that's switched over to 24 lb/hr; he may give you the old 19 lb/hr ones. Or you can check online. In general, fuel injectors are extremely reliable and usually do not malfunction (unless severely clogged), so if you get a good deal on a set, chances are you'll actually be getting a good deal. By the way, in case you're curious: if you mix the 19 lb/hr computer with your stock 14 lb/hr injectors, you will get a bad fuel starvation problem and driveability woes. You can't avoid getting the correct injectors for the HO conversion.
In order to work with the Mustang computer and injectors, you must also get a stock '87-'93 Mustang camshaft. It is the heart of the HO conversion. Ford re-ground cams almost every year for a little better performance and fuel economy, so stock HO cam part numbers changed, but they will all work with the HO computer and you will more than likely never notice any difference between them. It is therefore entirely possible to use a '92 HO cam with an '87 Mustang GT computer with no difficulties. Idle with the HO cam is very smooth, and top end is greatly increased over the stock Cougar cam. Generally you can use any 1987-93 HO cam with the speed density computer with great results. It is advised to avoid the 1986 HO cam as the later ones give a lot more usable power. There are lots of Mustang owners out there that would just love to get rid of the stock cam they have lying around (because they put an aftermarket one in)perhaps you can wrestle one away from them. One note of caution: since we have flat-top pistons from the factory, anything more potent than the stock HO camshaft may result in serious piston-to-valve clearance problems and/or driveability problems. The only other confirmed combination that works with flat top pistons is the Trick Flow cam with TFS heads. By using an HO cam, your firing order will be different from your stock one. The firing order will now be 1-3-7-2-6-5-4-8, using the HO pattern. You'll need to route the spark plug wires accordingly.