As far as the belts you have to be careful. I had this problem when I did mine.
The stock belt is a 119" "listed" and the ambulance belt is 119.5" listed.
I ordered two Dayco 5081195 which are listed for the ambulance package, at 119.5" and both were too short. When I physically measured them not only were they different lengths, but whey were both too short, one measured 118.75 and the other was 119".
I ended up ordereing the factory belt from Ford and they listed the belt as a 120" belt, but when I got it, I measured it for reference, and it was 119.5".
So I can only gather that the tolerances on the belts are not regulated at all, and the belt needs to be a true 119.5" and all belts are about 1/2" short according to their rating, so I assume the Ford belt is listed as a 120" so you get a 119.5" belt.
For reference, I probably could have put the stock belt back on, but the tensioner would a have been maxed out on travel. I marked the tensioner before the install and after with the correct belt and it was the same, so the stock belt would defitnately be too short.
As far the harness, I had a spool of 1 guage, so I just made my own.
Coincidently, if you do a search, this topic has been discussed a lot recently and you should be able to find all the answers you need.
Here is my 215 amp pics