As brion points out there are lots of things going on here..
I've just got this to go on
http://sailboatdata.com/VIEWRECORD.ASP?CLASS_ID=444 &
http://sailboatdata.com/VIEWRECORD.ASP?CLASS_ID=826
A double spreader is the way to go if the 'new' mast is a lighter section, but the ramifications in terms of complication are high. i.e. The bottom spreaders may finish up being quite low, therefore the lowers would be low and cause obstruction to the foredeck; not to mention the attachment for the intermediate stay to the deck, or make it discontinuous!?
brion has it, check the numbers (Distp. RM etc), find how close the moments of the 'ideal' and your 'new' rig are, and go from there. I guess it could be close, just on eyeball..
I would tend to think that single spreader rig would be the best, given the type of craft.
Cheers