Peter.....building a scratch built model boat from full size builders drawings and plans can itself be a little daunting, however attempting to gain sufficient dimensions and proportions from the internals of the real vessel would certainly be an added challenge especially understanding & correctly lofting the water lines
Is there a similar proportioned/designed vessel of shall we say
'near the same design that plans are available for' and could be melded to your specific vessel in mind?
I think this way you would save a lot of heartache
........... Derek