The board looks mean !
it's funny like Manta has the isonic tail of a few years ago and Isonic vice-versa....
I have a 2014 Manta 116. Compared to my 100 litre Tabou Speedster it does feel very boxy in comparison.
Mmy solution was to use a Sonntag 35 cm SLR with it - it rides much,much smoother now.
With a 7m R8 rigged full the 35 cm SLR fin is perfect as soon as I'm powered up on the plane.
Not powered up the fin does feel on the small side but only slightly. A 36 SLR could likely be perfect with this board,especially with a 7.8m.
I borrowed a 32 Sonntag SLR to try on my 65cm wide Tabou Speedster with a 6.2m and again it resulted in a really smooth ride. Awesome feel of control in the gusts and big chop. Trim was so perfect.
Smoothest slalom fins around - but you do need to use around 2cm smaller to get that soft smooth feel.
For a 106 Manta with a 7m I'd use a 34 cm in powered up conditions,a 35 cm in lighter wind.