You'll have enough room, when I replaced with Torx SS screws I used 1/2" everywhere no problem with miles to spare. If you run into a tight area, nothing that a cut off wheel and cleaning up the cut threads can't fix.
I've just started using rivnuts, however placement is key as there is no "give" once they are secured. Furthermore, I'm going to attempt to use some ss torx pan head screws and washers next go around.
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