You have the right idea.

Immobilize the fuselage on a large flat surface, and dry fit the wing.
Measure carefully to ensure a level, square fit.
If the wing is not level, adjust by building up the wing saddle with thin balsa shims and CA.
Don't worry about small gaps, you'll be filling them in anyway. Measure again and repeat.
Once everything is aligned properly, mark the wing position so you can put it back the same.

Now use the the epoxy and micro balloons.
Good luck with your project. This is an easy task. You'll be pleased with the results.