the thing is mate, you need the aic con bulkhead to chop out the bit that the air con lump of plastic sits in that houses the evaporator

the thing is probably 50% larger than your heater matrix and so the box it goes in is that much bigger as well, plus it's got the recycle gubbins fitted to the inside as well which complicates matters a bit more

i'll see if i can get a pic of one from the garage if i've still got it but it's a great big lump of a thing that only pictures will fully properly explain