If you're tail walking it's a sure sign of too big a fin.
One of the fins main purpose, apart from horizontal stability is to help lift the board out of the water so that it keeps planing. The faster you go, the more it wants to exit the water .. thus .. lift .. too much lift and you're tail walking.
If you take the 6.0 out on a light day, a big 34cm fin might be the go to get the board on the plane, but you won't be 'flying' along because with a big fin like that it causes drag which slows you down. Lift don't come for free!
With 103L of volume, the board is pretty "floaty" already, so shouldn't need much help to plane... would be great if you could borrow some different sizes to experiment..maybe a 28 pointer or smaller? Some of the pro's run down to a 26 pointer with 6m..
Another problem might be lack of mastfoot pressure .. when planing, check if your shoulders are further away from the board than you bum, and that your arms are straight...
..Laurie