Mopar does not recommend it's MPFI kit for older worn engines, as worn engines are prone to detonation with the Mopar kit. So, keep that in mind. You may also need to use high octane fuel to prevent detonation. Unless you MUST have the 4.2L, you're likely to find a rebuilt 4.0L, guts and all, and swap it in, for the same price (or less) than the Mopar kit and get the same performance.
With the Howell, what you end up with is an ignition system like that on older carbed engine (no spark advance control - you lose that when you ditch the factory Carter BBD setup), but you do get better fuel delivery precision than what the Carter BBD could ever dream of doing. I think of it as a sort of Frankenstein hybrid of old school carb tech and new school fuel injection. The install is less complicated than the Mopar. The guts of the Howell system are based on the Chevy/GM throttle body from a 4.3L V6.