Some other tricks to weld when you can't see what you are doing:
First: when you don't have a clear line of sight to the joint but, can see the back of the panel, what I like to do is to drill a very small hole at each end of the joint then, you go on the other side and you link the 2 holes with a scribe mark. Once this is done, you put the gun on the joint and you, being on the other side, watch for that red dot that the puddle will make on the back side of the panel and you move that red dot along the mark you made trying to keep it straight on the line. Voilà , blind wellding. If the dot becomes glowing red, let go or you'll burn trough.
When you can see the joint but you can't get the helmet close enough to see when it's flipped down, I like to take a mirror and put in in the hole instead of trying to cram my head in there. It takes a bit of practice but, if you can shave, you'll be able to pull it off.