Ironically, I'm working on the AT&SF FT-J as we speak too... but in N scale. 

You may find this helpful, PJ Student's article re the FT-J (and the Ft-O&P) in HO.  http://magazine.trainlife.com/wp-content/uploads/2017/08/ptm_197804.pdf starting on pg 16.
On pg 17, he states the side sill web plates (what Student call the fishbelly) are "inset 16" from the edge of the deck side sill."  No word on spacing of other bellies.  

Incidentally, would love to see other drawing or images of the FT-J.  (So far KSHS has not replied to my request to see their's listed on line here https://www.kshs.org/archives/67594.)

All I have to work with is the grainy Cyril Durrenberger image of the Wt-J in this Student article, but it seems to materially differ from the Student version beyond what he mentions, in that it seems to have 5 subfloor braces that pierce the side sill web plates and extend to flush with the deck side sills - perhaps 3 center as I beam-ish (center and other 2 at the point of the fish belly bend) and the two towards the ends being c or box channels? 

Also, interested in the width of the exposed side sill web plates, not given in the Student article, except as the width of the fishbelly, based on the dimensions of the Train-Miniature donor car, plus scabs he used to join the deck.  

Other that this drawing online of an Ft-Ia (apparently same as Ft-18?)  I can't help much here I've not researched it - but very interested as that is also on my to do list!  http://old.atsfrr.org/resources/zebrowski/folio/ft-pdf/ft-i.pdf

-Jake Schaible