I wanted to get bang for buck out of this awesome kit and be able to field either of the two models. This can be done rather simply with a few well placed magnets. Here's a quick rundown of how I did it and where all the magnets go.....
Parts
Here is the completed set of sub assembly's with magnets. More detail below.....

Back crew



 One in the rear of the plague monks and one on the "other" side of the plank.


Cog assembly
 shave off the plastic sprue nub, replace with magnets.

The hole in the chain had to be slightly drilled to fit my magnets.

Front crew
 One in the middle of the front plank, bottom of the plague priest, and warpstone censer basket. 


Completed Plague furnace assembly
 Everything that doesn't have a magnet just slides into place. 

 The only slightly tricky part was gluing this arm on. I assembled with the cog pieces, then glued the arm on. disassembled once good and dry. 
 I glued a piece of sprue here to prevent the back plank from popping up.

Completed Screaming Bell assembly