Oh Lordy, I now own a Fordy (a Lincoln actually)..

[big gear head]

The 9 inch chunk fits in place of the 9 3/8 chunk. You can't put a 9 3/8 chunk in a 9 inch housing because the ring gear will not fit into the housing. From the back side the 9 and 9 3/8 housings look the same, but if you look at the opening in the front you will see that the 9 3/8 housing has notches to clear the larger ring gear. So any 31 spline 9 inch chunk will fit into your 9 3/8 housing. I have a 9 3/8 Traction Lock chunk, but I can't remember what the gear ratio is.