Rear cab seam rust

Hi,

Any suggestions on removing or dealing with the rust in the seam in the back of the cab below the back window of my 51 chevy. I know that idealy the roof section should be seperated from the lower rear section of the cab and cleaned up but maybe there are other easier ways.....

Thanks in advance for your help!

I had to deal with this on my 49' and i sandblasted as well as i could and sprayed the metal with a rust converter (not sure of the brand, bought from local agri-supply) it turns rust black and seals itself. i also had to spot a few holes up here and there. then body filled and primed. paint has been on for about 6 mos. so far so good