You might want to check into a Howell fuel injection kit, instead of the Hesco. The Howell kit comes complete with everything you need to make a 4.2 fuel injected.
As far as the header, there will be tons of clearance problems, with the front driveshaft, not to mention that you will have to make a custom exhaust from the collector back.
I made the ignition upgrade when i put the new long block 4.2L in my 87, HEI distributor with an Accel super coil. It isnt really needed, the stock stuff works just fine, I did it cause i wanted to.
The things that you are missing are fuel delivery to the MPI system. It is the only thing that is keeping me from putting a howell system on mine. The stock pump(on the motor) wont work (wrong p.s.i.), and you cant switch over the sending unit in your tank to a fuel pump unit cause the hole is a different size. So, you either have to put an electric pump somewhere on the frame(which isnt cool, cause it would be subjected to the abuse of off roading) or...replace the tank with a newer model that will hold a pump/sending unit in it.