Jimbo, Welcome to the forum.
There are two schools of thought on locating the front trunnion, I have used both, and both work.
A) use your forearm to locate the trunnion, make sure the front of the receiver measures 1.200 in height. temporarily bolt in the selector stop and trigger guard and check mag fit.
B) Rivet in the SS and TG then use a mag to the locate front trunnion. Then check forearm fit.
The whole trick is to get the correct spacing between the trunnion and the mag catch.
As far as the rear goes, thats easy, once you have the front located correctly, use the top cover to locate the rear.
You didn't mention what type of torch set you have but I'm assuming it's Oxy/Acetylene they will work fine for heat treating your ejector and axis pin holes.