On the belt line of my 54 if I remember correctly I used 3m brushable seem sealer was done in1988 still on there although it is not on the top like yours in SoCal but in snow cal Ontario .I believe you can get the same stuff in a tube for easier apply .I used masking tape to tape off the area were I did not want it and used the rest for sealing were I welded patches on the underside .
