This is likely no mystery to anyone but me, but for weeks I've been perplexed by comparing the aforementioned PJ Student's article re building an Ft-J (v similar to the Ft-I).  Again, Student failed to include the 5 side sill supports that can be seen peaking out from under his article's grainy Cyril Durrenberger image of the ATSF Wt-J 2027?7.  My question is were added on later with one of the several rebuilds?  Or were they original, and Student just ignored this feature in his models.  

Charlie Slater was kind enough to share images of these feature, which show they were not - as I initially suspected - contiguous parts.  Rather they are compound built up plates riveted to the floor support / cross bar features, which seemed for a bit to bolster my hypothesis (and hope) that they were added in one of the later rebuild phases.   Other late images of converted Ft-I cars show these better:   https://upload.wikimedia.org/wikipedia/commons/c/c3/Atchison%2C_Topeka%2C_and_Santa_Fe%2C_Flat_Car_97834_%2815894678128%29.jpg

Torn on how to advance, I broke down and finally purchased a book long on my wish list : Hendrickson's "Santa Fe Open-Top Cars" (on sale for just $20 now at https://sfrhms.org/product/vol-7-open-top-cars-flat-gon-hopper-cars-1902-1959/)  No excuse for waiting so long, but at ~1/3 the list price?  Sold!  Anyway, reviewing the Ft-I & Ft-J cars (pg 25 - 28) all show these 5 side sill support features.  Only exception is a converted Wt-J ATSF 191240, caught in 1954.  However several feature of this car (including 8 stake pockets instead of 10 in the as built Ft-I &Ft-J) have me doubting it was part of the Ft-I or -J classes. 

So .... shoot.... now I need to make and fit 10 side sill plates per car... in N scale!   ;-)