Sealing roof

I need to reseal the roof on my series 3 truck cab and although I’ve read various old posts I’m not sure what to use or where to seal. It’s mainly leaking through to the interior gutters and the rivets look ok. I was thinking of running mastic down the inside where the roof meets the gutter but will this give me a problem if I want to remove the roof later on?

If you seal the roof how will you get enough water in to rot the bulkhead and lower dash panel?