Anyway will body panel adhesive hold up to the flex between the the fiberglass and metal? Would I need to bondo the seam after joining with the adhesive or use something else? Exo is in the plan but it is going to have to be after paint.

Thanks for the ideas

I have never owned or worked on one of these, and was under the impression that the weather striping covers up the rivets used to hold the fiberglass cab to the original cab.  If that is how it is, the body seam adhesive will stay pliable enough once cured that it wont crack if there is any movement between the two.

If you are wanting to make it a seamless transition, that could be quite a lot of work.  Going off the pics, I would think you would want to pull the two sections apart, use body adhesive to bond the two together, rivet(if that is what was done originally), then sand the edge of the fiberglass before working the seam and old weather strip section with body filler.  Before trying it though, I would swing by a body shop(preferably a custom one) and see what they think can or should be done, the work it is going to require could be useless if you can't make a bond that wont suffer from vibrations, flex, or difference in how steel and fiberglass expand/contract w/heat and cold.

Don't use "Bondo" from the auto parts store, it shrinks a lot, you want to use the better stuff from Summit Racing or a local paint store that supplies body shops, it doesn't cost that much more, wont shrink that much, and it sands down a lot easier as well.