That's the problem with panel bond on the outside of a car. The seam raises up when the materials expand and contract at different rates, but there will be those that swear it's the best thing since sliced bread.
OP said no welding equip available. I was going for the safest almost correct way of doing this. brass sucks big time trying to prime over it, I'd use a map torch and lead it over the plug first. Paddling lead was my very first job at a body/restoration shop and it ain't really that hard just finicky on how many seconds you have to work with it in the correct stage of softness and properly cleaned and tinned is a Must.
I'm not telling how many tries my boss made me do on a junk panel but once I found the sweet spot by looks it was easy. (just didn't know about all the toxic stuff so be aware of it and wear a GOOD respirator and gloves.)
I would not think twice about using panel bond to put in floor pans and structural areas and you are right about the difference between glue and steel expansion so it makes it hard to recommend it for that, but if you can get a really tight fitting plug of metal with a larger backer that is glued to the plug you can minimize the expansion/contraction if no glue goes into the plug/hole area, yeah that is hard to do but it can be done and then finished properly it helps.
