1803 installer always stuck at 43% on my old home built with AMD Radeon 6850. I finally made it work by applying kb4057291 and reinstall AMD driver. However, I don't know which one exactly helpd, or both.
If this is still an issue, I suggest to reinstall AMD Radeon driver directly even it says the same driver was already installed. Once video driver is re-installed, try 1803 installer via Windows update assistant. If still not work, try kb4057291.
Good luck!
Jin.