Going back to this, I've found a forum on where the guy made these graphics/models of a SJP with extended Gallowgate End and East Stand. If the Gallowgate was extended, it would take the capacity to 61,188. And, although it's pretty much impossible due to the listed buildings behind it, if the East Stand was extended also, the capacity would be taken to 81,219. Thought it was a quite interesting discussion.