The boat in this post have in line spreaders..
The boat originally was designed with in line spreaders and you are going for swept spreaders you must revise the chainplate / mast location.
If you are switching from Continuous to discontinuous rigging it might be possible to use the aft chainplate for the shrouds lower vertical and keep the mast location.
But you need proper engineering to make the right decision.
You can read more about this here http://www.sparcraft.com/uk/products.../spreaders.pdf
It's kind of curious that Selden and the rigger seems to disagree, who specified the new mast?