Brian,
I did this just last year and so far, it is working out fine with no leaks. My fiberglass cap needed a lot of repair, so while I had that out, I ran a 2" wide piece of glas tape around the entire edge, essentially glassing in the old holes, I had some rotten spots and tears where the old rivets were. I also welded up the corresponding holes in the driprail. I POR'd the driprail and did a trial fit of the cap. Then I used silicone modified latex caulk, like DAP and ran a nice bead around the gutter where the cap mated to it, then layed the cap on and weighted it down just so it would completely and evenly seat onto the driprail. I didn't want to squeeze out all the caulk.
Let it dry for 24 hours, then I drilled new holes halfway between the old ones and used pop rivets with the backing washers. Riveted from the bottom side of the driprail so the washer and squshed side of the rivet are on top. Then laid a bead of caulk in the trough to smooth things out. You can use a wet rag or your wet finger and a rag to smooth it. That's why I like the silicone modified caulk, water cleanup and it is paintable. The rivet holes are not really an issue since they are outside of the weaterstrip underneath.
Clear as MUD?
GL
Ed Long