Prime or paint before reassemble?

[LMS1967K10]

Ok just want to know what you all think we should or shouldn't do. We have my sons truck stripped down to the frame...now parts are all over my garage...old on one side and new parts in boxes on the other. Everything on the frame has been replaced and or rebuilt then repainted with high gloss black epoxy. Now I would like to start reassembling the truck...My question is say if I reassemble the bed with all the new parts should I prime or paint a base coat where the panels meet up or just trust the coating on the new panels?? What do you do or suggest? Thank you!