id recommend the 507k for a 43x. a 507c will sit very high with the adapter plate needed to mount it because a 507c is much wider than the slide. the width of a 507k is about equal to the slide of a 43x. you can certainly do it, but a k footprint optic works much better
There's a good reason to call The Gun Butler and ask him what optics can be cut into that slide as then no adapters will be needed and it will sit as low as possible.
yes sir. getting your slide milled for your optics footprint is always the best route to go. stronger and sits lower.
a 507c is far too wide to have a glock 43x milled to direct mount it though. a 507k isn’t though. you can see here just how much wider the 507c on the right is than the 507k on the p365 on the left