The receiver below is really embarassingly dirty- as I have not cleaned it after it was cut/demilled. I am no expert at all with building these but hopefully this helps-please consult with others more adept at this than I! There are some cats on hkpro that could help I'm sure.
I believe the best thing to do is to tack the trunion in place first?
The trick is to get something rigid/appropriately shaped and non-marring into the receiver to press against the trunion face etc. etc. Pressing the barrel into the trunion until "close" before tacking makes sense to me.
I can see merit for pressing the barrel from either direction... I typically like to freeze the pressed piece and have the receiving end at least pretty warm to help with getting a good square start and to minimize the possibility of galling.
I suppose "worst" case is that you have to grind off the weldment/remove the trunion/barrel and buy another trunion and start over if something goes awry like the barrel is pressed in too far or ??

standard rollers are typically not marked on the smooth end, +2/+4 or -2 etc. rollers are usually marked, sometimes with just dashes vs numerals. I believe the standard rollers should measure 8.0mm diam. You will need feeler gauges to measure your bolt gap.