I would look outside of the Tyrannofex kit to convert a different pose. The legs are too thin and the body to long and front heavy to look right in any other pose.
As an example of going outside the kit look at my Exocrine.
Head and torso are carnifex, front legs are tfex walking legs with an extra joint made from a large pair of scy tal. Rear legs are toxicrene. Tail from a harpy. Bioplasmic cannon made from the tfex kit. Blades on tail and head came from Tyrant Guard Scytal hands. Venom sacs for the eyes on the gun.
Tyranid bits mash together REALLY well with just a little bit of green stuff to smooth out the edges.
Consider what kind of profile and scale you want to go for, find bits that match that and use them in a mash up to make it your own.