This may not be exactly what you need but could help. When I was researching purchase
of an E40 a few years ago I got these tank replacement / repair photos from a member
on the Endeavour
Owners Forum. Unfortunately it's not real active lately as you've found.
Anyway, to best approximate the tank size this picture at least shows you the void space after the tank has been removed. If you can measure the depth
of your keel
pocket that will give you a ballpark figure at least. I remember learning
that the original tank plans etc were held by FMT as you've discovered.. several owners had purchased replacements
Sorry I couldn't be of more help. I've long ago lost
address of the gent who sent me these photos...