Any of the brackets that I've seen have been spot welded to the sub frame, however there's usually holes there too, so they may have been riveted on in the earlier production runs. Being as the bracket is eventually totally out of sight, any other method of attachment you may want to use shouldn't matter as long as it's sturdy.
Here's some pictures of a cowl where the sub frame and the bracket have been cut through - should show you what you need to line things up. Also an underside view of an original sill panel which was wrapped under but not crimped to the lower outside edge of the sub frame. Main consideration here is to try to get the reveals on the cowl, the door sill and the rear panel to line up.