euh.
now that you got your IXD2 tower to look like a type XVIII tower (

)
you have to edit your IXD2 flak guns to be XVIII flak guns. I do believe that would require editing the UnitParts1IXD2.upc file in Data/UPCDataGE/UPCUnitsData
it's an educated guess and I'm a little busy with other things to instruct you further. I hope it ends up working for ya!