There is a description from the photographer about the process and other postings pointing out some features at
http://news.ycombinator.com/item?id=5256771"That is 96000 images with not a shot missed"
"Stitching ended up taking nearly 4 months when we thought it would take about 4 weeks"
At this level its not just classic stitching software you need as the images are almost flat in the limit of the image sphere. It is more managing the enormous datasets as you already know the sequence and angle the images were shot in. It is very impressive.
When i read the processing for the Hillhead underground Alisdair Gray image took so long i would guess they were using photoshop rather than something like vips
http://www.vips.ecs.soton.ac.uk/index.php?title=VIPS which is more the tool to use.