I completely spaced on Summers Brothers, in Ontario, even though I have a pair of their axles in my Opel nine inch. (Frankly, I wasn't sure if they were still in business)
I DONT think they do the narrowing, but since they have been supplying custom axles for about 50 years, I bet they can recommend a shop.
They were about 4 miles from my house. Unfortunately, they have moved about 4 miles further. They now seem to specialize in off road vehicles and other racing item other than axles. But your suggestion makes sense! I will call them tomorrow!