Swapping a 4.0 FI motor still requires the wiring harness, computer, new fuel tank, high pressure fuel lines, CPS on the trans, new alternator, and new smog equipment plus other mods.
The easiest and cheapest way to go is the Howell FI system like Chris mentioned. It's a complete kit, direct swap for your carb, comes with the computer, CA smog legal, and you can use a external fuel pump. I think it comes with it.
The Howell kit is basically a new manifold base plate you put on your existing 4.2 stock manifold and it uses an early nineties Chevy 4.3 liter throttle body. They work really well and are simple. Couple hours and you are done.