No no no it is very easy, please give it a try :p Yo could do only exterior and reuse the interior...Salmon and Sargo appear as different subs but could be regrouped in a category or upgrade, or just let one of them out, leaving place for the Narwhal
Also, if the models already in SH4 can be edited, probably doing a Narwhal starting from a Perch or Salmon would be much easier....
