without a pic it's hard to see what you got goin on there
but from what i'm hearing you may want to cut some strips
of sheetmetal to cover the "gap" (like a long band-aid)
quality seam sealer of some tuff stuff
brush on some to the gap, lay the sheetmetal strip over the opening
and brush a couple coats over the top
(don't try to pile up seam sealer too high it won't cure completely)do it in coats, it don't hurt to even let it set over night between them
there is alot of car building going on with "no weld panels"
and it's all put together with seam sealer/glue
i did a door panel the other day...
well ok i did a window regulator the other day
but had to remove the outter door panel to get at it!
the whole thing was glued together
-
most important is remove any old rust before doing anything
and it'll last a long time