cheapest way to get hemi 3g going?

Found this.
[ame="http://www.youtube.com/watch?v=U_y4xuIVajQ"]9-29-12 how is Megasquirt wired into a car - YouTube[/ame]

my basic understanding from this video and others...
1. need something to fire injectors (megasquirt).
2. need something to fire spark (msd box)
megasquirt picks up signal from TPS, few other sensors and can tell the injectors when to fire.
MSD does its thing some how and tells coils to fire.

Now those running carb setup, say they have to have an MSD box too cuz there is no distributor and its pretty much just plug and play.

So what makes this harder? I'm sure im grossly over simplifying this process....but why do i need to get a harness hacked and what not? can't I just plug megasquirt into the fuel injectors and get it working?