A few things here. 3-m 8115 adhesive or equivalent can give a strong crack resistant bond and where it covers bare metal it will keep rust out. Bonding edges of bare metal need to be sanded coarse for good adhesion. Contact area around perimeter to be glued needs to be a decent width for glue to be able to do it's job.
Though epoxy will bond fiberglass to steel, they will expand & contract at different rates which in some cases can cause harsh shadowing in the future. If a fiberglass part has a bit of an intersecting angle creating a bodyline, it can mask the shadowing effect pretty good. If a fiberglass part has to lay flat & blend into a steel panel, (top of your part it looks like) that will more likely than not shadow. A filler such as SEM carbo fill as the base coat of body filler can minimize this effect.
Some fiberglass kits are better than others, average ones for imports probably aren't near the greatest quality. That said, with any fiberglass parts, your best friend is to let the parts get at least a few days minimum of hot sun after bonding on before final bodywork. The lesser quality parts can shrink a lot in the first few weeks.
An option for the wheelwell portion of the bond, which won't be highly seen, but gets the most stress, would be to use a commercial urethane caulk/ sealant instead of the stiffer more likely to crack epoxy. Using a caulk at my day job past couple of years from tremco called Dymonic. It's passed several personal stress tests I've put it through on metal to metal & metal to fiberglass. Will be using in wheel well to quarter panel bond on my steel widebody project. Cheap enough to use a lot more of it to ensure coverage & a wider bonding area. Yea, not traditional automotive product, but I think you just heard enough opinions about traditional stuff sucking.
https://www.tremcosealants.com/fileshare/DataSheets_Hyland/Dymonic_100_DS.pdf