Tip is to apply copaslip to the thread and not overtightened them. I never did mine much more than finger tight. The rubber gaskets hold them in.
I was talking from a 748/916 perspective. From 749 perspective, they appear similar but cannot guarantee length is same.
Never mind the fasteners you need to be fkn Octupus with a mate who’s a Spider to fit those mirrors. Whoever designed them needs a good shoeing, kunt.