So the photo is an example of where I did this successfully before. I remember using either BlendSrf or MatchSrf, but it just isn’t working in this new design I am making. The issue with MatchSrf is that this tail wing is a polysurface and I spent hours trying to turn it into an untrimmed surface with no lock. BlendSrf is giving crazy messy results, even if I cut the wing half way up.
OK, but I wanted to understand how you expect the blend to work: swooping up from the significant lump under the fuselage or cutting it off in line in some way. It is easier to give advice about a specific scenario and I’m time poor at the moment.
But in general terms, I think that as a first step you would be well advised to build new surfaces that are way simpler than the ones you have at present, particularly the flat areas. That should make the blending process easier to manage.