Sometimes you just need to sleep on things. It has been too long since I worked on a construction site. I have been worried for several days about the problems of getting the two sides of each voussoir properly registered. Of course, the only way is to match build them, and preferably to build the whole arch rather than just matching pairs together.
Tomorrow I will try to finish the abutments as they will be needed. On Monday, if we can find a flat floor to do it on I will set up the abutments, properly spaced, then fit sheets of ply or other board along the outside where the arch faces are going to go. The faces can then be temporarily screwed into place and the cross pieces positioned in situ. It will also be useful to fasten on the deck while the vousoirs are up there to make sure things stay square and parallel.
That should be the last tricky part. If we can get the pre-assembly finished on Monday, the rest can be left to the team and I can take a rest.
If I thought I could draw this in a way that was understandable I would do so. I think that, if you don't understand the above, you will have to wait for the photographs.
Thinking as I write, it might be easier to make a frame on which three voussoirs would sit properly aligned. Then we would build three together, move them along one and build the next one on the end. Again, I guess a photo is needed, but not till it has been done.