After realising that laying each spiral individually wasn't the smartest method, I decided to make a spline that would do the job for me. It took some fiddling around with numbers to get the start and end meshes lined up perfectly but I got there in the end. I now just need lay this spline along a centre edge of the arches and it lines up perfectly as needed. The two variants of start and end mesh are because the pillars on the right are a mirror of the ones of the left so multiple meshes are needed to recreate the exact pattern of overlapping spirals from the reference.
Once happy that the spline would work, I loaded everything into UE4 to see how the scene was coming together in there. These images still show the old arches as I will be updating all of them shortly with the new spline. I've tried to recreate the exact shots from the film here which helped with checking sizes and layouts too.
Next, I will be back working on the arches. I'm going to bake the spiral details down to a height map in Marmoset and use Parallax Occlusion Mapping for them in UE4. This should drastically help with reducing the tri count, especially from high up areas that a player would never be expected to see close enough if this were used in a game. I've never used POM before so this will be a bit of a learning curve but it will kickstart more work on the technical side of the project and also give me a base for my material setup which will be needed further down the line once the whole mesh is complete.