I remember a discussion back when I purchased my copy of this kit a long time ago- there's some good links in it, including some links to nice 'walk-around' photos.
/forums/t/122460.aspx?PageIndex=1
After much consideration and googling, I figured that the armament of D.Fernando kit depicts her 'design capacity' and is probably not representative of the ship in her role as a colonial/ diplomatic vessel. I would be very suprised if she regularly carried such a press of weaponry when she also had half of her gun deck given over to cabins. The restorers of the ship seemed pretty sure that the spar deck armament would have carried 20x 18 pound carronades and two long-gun bow chasers. That seems perfectly sensible given the sheer weight the ship must have carried on diplomatic missions. I bet she was a nightmare to trim and I wouldn't be suprised if I read that she was a poor sailer!
Much of my research seemed to indicate that original technical records and plans of the vessel were almost non-existant, certainly from the period of her build. I guess most of the decisions, even over fitting- her out would have been made out in Goa rather than in Portugal.
The best information I could find online is a page discussing how decisions were made as to the museum ship's appearence.
http://www.prof2000.pt/users/avcultur/fragata_d_fernando/06Apetrechamento.htm
On the deck plan contained in the above link, note the cabins extending along the gun deck for 1/3 of it's length. and the lack of guns (either carronades of long guns) along the area adjacent to the gangways (not that she has gangways in any real sense). Not much work to depict this using the Revell/Zvezda parts.
Will