Not sure how the Stern widebody's worked back then, but if you are doing anything except a small quick fix I would disconnect the playfield and work on it outside of the cab.

So much easier to work on things at a table or on a rotisserie.

To attempt to answer your question.. it looks like the bottom edge of the playfield would need to go up and over the metal things you have circled to rest against the wood where it makes an 'L' then you should be able to lean to the backbox. Again not 100% and can't see everything from your pic, but that would be my instinct on what to do given what I can see.

Yes, you can pull the playfield past the brackets and rest it on the wood supports. I don't slide mine forward and let it drop, though. I lift the playfield up and over the brackets and set it down on the wood.

Maybe you could take the brackets off, move the pf, then put the brackets back on when you're done?

Yes, this is the way to do it. Some of the harnesses are tighter than others, but I haven't found one classic Bally/Stern that couldn't be worked on in this fashion. Of course, Centaur and some others have pegs where you can rest the playfield instead of those wooden shelves.

Keep in mind, it is heavy, and will be harder to put back over the metal brackets when you are done.

That's a good idea too. Another thing you can do when you slide the playfield out is get your hand under the top end of the pf as you slide and support it so it won't drop down to the wood rails.