Yep, been busy this month...
OK, start with the standard Zep scaffold.
Unbutton the A/C shroud.
The exposed original A/C looks like this. If you remove the entire A/C unit, including its mounting plate, it's too heavy to manage by one person. If you're doing this alone, remove the compressor, which is half the total weight of the A/C--four bolts and cut three copper pipes. There's no helping the possible release of freon--you can't get it off by yourself.
You can remove just the components and leave the mounting "plate" on the roof. I recommend this approach if your plate is attached all around with solid rivets. Second, if you keep the plate, you have a flat surface to mount your new A/C. You also avoid having to seal up 50-some rivet holes, most of which require Olympics, since the interior skin prevents back side access. On the other hand, the removal of the components will open up some other holes that will need to be sealed.
Note the flanges around the edges of the A/C "plate." These stick up high enough to be unsightly from the ground. You can either bend them down flat onto the top of the plate, cut them off with a sawzall, or, if you're lucky, you can break the spot welds and remove them. If you bend them over flat, you will have a stronger plate.
Anyway, this one was attached with pop rivets so I removed the whole thing. I sure like the looks without an A/C...
Whether or not you keep the plate, you MUST construct internal braces that counter the squeeze forces from the bolts used in the modern A/C attachment method. These braces only need to be attached to the outer shell. The internal ceiling skin is not structural so screwing the internal skin to the braces only adds a small amount of support. With only the outside connections, the braces are very stiff--you could do a pullup on them.
You, not me. The braces do not need to be attached at the corners--remember, they are mostly there to take compression loads. In this Overlander, there is a rib at the front edge of the opening, so only three pieces of wood were needed.
You can see the pattern of the old plate's perimeter rivet holes here. I used solid rivets where I could get at them and Olypmpics along the street and curb edges.
Have fun. You can do this by yourself except getting the new A/C up on the scaffold. If the ladders didn't project up above the platform, you could slide the new A/C up the ladder and plop, onto the plank, but not with this scoffold.
Make sure you remember to route the power wire through the braces. Also, I put the drain line in there so that it's accessible--not needed on this model A/C, but some do need it.