I spent many hours searching the internet for accurate dimensions to at least be able to accurately locate the front and rear frame rails. I felt like I could manage from there. There was a dimensioned drawing in the Ford shop manual, but it had some incorrect dimensions on it. I was eventually confident enough to make a jig from 2" square tube. I did a LOT of measuring and leveling with a surveyor's auto level, and I learned not to do anything more than tack weld as I went because each piece required small adjustments. In the end, all the structural pieces came together really nicely.
I started off trying to use a Harbor Freight spot welder where I could, but that thing is very heavy. I also wasn't confident that it was working properly. So, I mostly made plug welds with a mig. There are tables that show the proper spacing for spot/plug welds, based on steel thickness.