What is exactly the problem ? The lacking brush to form the arch top or the textured alignment ?
Using QuArK, I'd solve the first this way:

Most of the time, snap to grid is your friend.

This example uses a 8 qu grid, which works fine for most maps.
The misaligned textures are more complicated, I don't know any way to reliably make them look right without a lot of hand tweaking.
(that's why most of these cases I'd go for some texture that don't require alignment at all)