Wiring questions

I have alot of questions that I need cleared up. I got an 05 5.7 hemi going in my 71 Scamp with a 727. I'm looking into all the wiring and stuff and it is my weakness, but I'm trying to learn/understand it. I know I need a harness but that's about it. I was looking into a mega squirt, but I got confused on weather if i get one do i still need the ecu out of the truck i pulled the engine from or do i need just that. Also do i need an ignition box with it as well. Does the harness connect to the 727? I have more but I'll have to think of them as I go. Pretty much looking for someone to explain briefly what hooks to what and kinda how it work. This is my first engine build by myself and also my first project car fyi.