You don't need to change the fuselage length at all. But I would plan on making the tail surfaces larger to make up for the increase in wing area. The tail surface areas should be increased by about 20% each.

If you make it much wider without making it taller it'll look odd if you do it too much. A small amount would be OK though.

And don't forget that the upper wing struts will require some hard points and spread out structure added to the fuselage to properly support the upper wing loads.

Sounds like a fun project. Be sure to post up a build thread in the Scratch Build forum since this won't be much of a kit build anymore.