I actually purchased a CNG kit for my 5.9L Magnum just last week. I am going to do some serious maintainence before I install the kit. I opted against sequential injection system like you have described due to cost and the fact that my neighbor got his 5.4L Ford Triton motor to run fine on a mixer kit.

Here in Utah, we generally enjoy the cheapest price on CNG in the US and conversions are popular for gasoline engines and a handful of people do CNG injection in their diesel trucks.

I bought my kit at ewsews.com