As this is the Plasma Cannon rather than the Assault Cannons of the Talonmaster conversion I decided not to do an underslung Cannon and instead have it mounted in the chassis similar to a Scimitar pattern Heresy Jetbike. I went looking for the Forgeworld link for these when I started writing this post and couldn't find them, a couple of days later they're announced in plastic!
The Plasma Cannon is from the Deathwing Terminator Command sprue, so could have been underslung with ease but I wanted a more streamlined profile, so a bit of cutting and digging was needed to fit this in.
Here's the underneath where I had to remove the front bit of the central piece to fit the cannon.
I have a small metal ruler that I aligned with the three grav nodes to score along the base. I cut out most of it, but you can still see the score marks and the piece with a central node I glued back in to close the gap. It's pretty messy so I've got to clean it up yet.
I don't know if you could have just cut the small piece out that needs to be removed to fit the Plasma Cannon, without taking the piece I returned but it might be worth trying.
Mold lines need cleaning. You can also see that the bottom part, where it joins the 45º lower left gun cowling has started to separate. Not sure why but it'll be fixed. Fitting the Storm Bolter in the gun cowl will be a challenge. It comes as a Hurricane Bolter [3 guns] I can remove one which will make it a Storm Bolter and allow it to fit better because the Cannon currently resides where that third lower gun sits. There's some fettling to do that's for sure.
I've got my rider and extra bits to try and fit in the next update but that's in the post...