Update: I've attempt to bond the ribs to the roof with 3M 8115 panel bond - only a single section on two of the roof ribs had come loose, and so even though I knew that 8115 is not the OEM specified adhesive I took a chance. It seems to hold, and while the roof is still not quite as flat as it was when I started, it's a significant improvement. Time will tell if it hold up and if there are any NVH concerns. 8115 is NOT intended to be used on roof ribs, and I suspect if you were to use it to adhere all the ribs you might have trouble with stretching / cracking the sheet metal.
3M 04724 appears to be a better solution for re-bonding roof ribs, but I didn't learn about this product until after I had applied the 8115. Hilariously, 3M suggests using 04724 in the product manual for 8115 but I had missed that section. I'm not sure than 04724 would've worked in my application, since I need a reasonable amount of bonding strength, but time will tell.
It's possible that I've made a mistake and will put undo stress on the roof by applying 8115 but the roof beam is still able to flex a fair amount, the rest of the van is put together with the correct NVH foam, and I suspect everything will be OK. If the bond fails, I'll update this post.
I was not able to gain enough access to remove the existing NVH foam, so the 3M 8115 went over that. I also was not able to remove the paint on the ceiling - which means this is a less than ideal bond. I'm letting it dry under pressure (2x4 supporting the rib) for another day or so before "road testing" it.
8115 is really interesting stuff! I'm excited to find a better application for it.
2011 Ford E250 EB Cargo
5.4L V8 -- 3.73 gears -- RSC