Personally, I would make the fit fit the best you can, between these panels, and thoroughly apply Epoxy sealer along that mating surface, then use windscreen mastic sandwiched between. This will allow flex, movement whilst still holding firm against the mating surface.
If not! The vibration of these panels will wear the paint down, so you end up with creaks and squeaks, then the dreaded rust will start over again as it will bare metal in areas.
I have used 3M protective tape along both edges on a different cab, but same application, the tape just wore through to steel.
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John
EK Holden V8
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