I have dry fit this model a bit, and I feel silly that I have to ask this question. How do you put the guns on top of this beast?
I must admint that I didn't even try to assemble the beasts as Mutan, mine are both Surda Symbiobeasts.
I also assembled both mine as Surda. But the guns are one piece connected by a crossbar, and you slot the crossbar in from behind into a nook in the spine hump.
This was really unintuitive to put together. It doesn't fit very well and I ended up using a lot of green stuff to bolster it up so the guns wouldn't fall off.