What is the standard procedure for positioning the trunion and transfering the hole locations?
I'm using OOW receivers.
Make a jig using 2 old hack saw blades riveted together at one end. Put a rivet or small bolt in the other end and when you put the rivet in the hole in the truuion the top blade will be lined up with the hole underneath if you slip them together.
This is the method I used when building my Bulgie AK74 kit.
I'm an aircraft mechanic so I have alot of sheet metal tools which helped in the build. One of the tools of my trade is called a Cleco. Cleco's are used to hold parts together when riveting.
The Cleco is inserted in the holes of the two parts to be held together using Cleco pliers, which open the jaws of the Cleco when inserting it in the hole. The pliers are released leaving the cleco to hold the parts, this way the next hole can be match drilled.
I matched up the old Bulgie receiver piece against the new OOW receiver then scribed the location of the first hole into the OOW receiver.
I then drilled the hole in the OOW receiver. Next I used the Cleco inserted into this first hole to hold the old receiver and new receiver together. I then match drilled the remaining holes.
You could insert a bolt of the proper diameter into the first hole, followed by a nut to hold the two receivers together, then match drill the rest of the holes.
Buying Cleco pliers and proper dia Clecos is not necessary, but I thought if I explain my method you'd get the idea.
The Clecos did help alot though!