[84LX89GT]

It's pretty easy if you want to use a HO firing order 302 on an original non HO equipped block. Buy a 302 HO/351W camshaft. The '83-'85 302 HO mustangs were flat tappet blocks like yours and had the HO firing order that your computer would like to see so the injector timing would be back to where it should be and for ignition timing all you'd have to do is re-route the plug wires to the correct firing order. All you'd really have to do is replace the cam and lifters (flat tappet lifters break in with the cam). you could keep EVERYTHING else the way it is. Swapping the camshaft isn't that hard anyways and if you're putting on an EFI manifold anyways, you'll be inside your engine that far so it's not a huge deal. You can keep your stock pushrods, rocker arms, etc. etc. One thing you may have to look into is what kind of distributor drive gear you'll need with the camshaft (you may have need to change it anyways) because the distributor may have a harder/softer gear than the camshaft. Those are the only issues i can think of off the top of my head.
Good luck,