There are a few bits packs from forgeworld for upgrading chaos terminators to cult terminators.
Typically this involves replacing the front chest plate.
As such there tends to sometimes be plastic chaos terminator chests on ebay, as these are left over when using the forgeworld kits.
My recommendation would be get some of these cheaply as they will fit on to the dark vengeance terminator bodies (and obviously the chaos heads are designed to fit into these).
I did something similar to this, but I did not use chaos terminator heads.
Take a look how the chaos fronts look on
DV terminators:
The only thing is you may want to cover the gap between the halo and the 'backpack' (I used a spike from a
csm shoulderpad and a chaos spawn eye).