1985 Stepside bed paint rebuild

[jcwoodworks]

I am rebuilding a 1985 stepside that I picked up in Missouri to replace the rusted out fleetside on my 1977. The bed is totally apart and the repairs are in progress. My question is, when attaching the fenders to the body side, do I need to add seam sealer or a fender welt between the two? Also this is my first time doing the total paint. Shoud I prime, assemble then final paint and clear OR should I prime, paint, clear then assemble. Any suggestions are welcome. Since I am trying to do this properly to prevent as much rust as possible I want to do it correctly. I am repaiing all rusted areas, epoxy primer, block or high build primer, paint then clear? Thanks.