Depending on what your agenda for the guitar is - this technique was first mentioned to me by a professional instrument maker. Superglue (cyanoacrylate ester? stuff) is thin and runny. It will naturally work itself along the crack and provide a bond between the two surfaces of the wood. This is about the only thing I have found it to be useful for - blowed if I can ever persuade it to stick anything else together - I've tried to stick my fingers together as advertised, but only partially managed that one twice in twenty years of trying!
Problems you may experience with this technique would be :-
GLUE CREEP (this is NOT an instruction to get rid of somebody you dislike) - Superglue seeps out of the crack onto the surface of your soundboard. If the board is already varnished it can make a mess of the varnish. If the soundboard is unvarnished, you should be able to sand the glue line flush with the wood, in which case it simply looks like natural resin in the grain.
If there is any oil in the crack, the glue may not bind to the surfaces.
I have a mandoline which got dropped and the soundboard cracked. The superglue technique has held it (there is no stress or pull on the joint) for many years without noticeably affecting the playing tone. There is a slight glue line on the varnish, but I can't be bothered to attempt to tackle that cosmetic blemish.
Quack!
Geoff the Duck.