Are they assembled or on the sprue still?
The Chimera comes
wth all the parts required for the Basilisk, except for the gun and the support plate. You can easily create the support plate out of plasticard and then fit any large artillery piece you can find into the bed of the tank.
If they are already assembled, then you would need to trim down the rear compartment in some way to open it up. Again not too onerous a deal
a good example on modular chimeras here
http://www.dakkadakka.com/dakkaforum/posts/list/0/438724.page
Also I think Ailaros did some modular Guard vehicles with lots of plasticard.