Technical Converting a parallel axle to rear mounted transverse spring.

Conventional Ford axles have a boss cast into them, where the perch bolts go. One like yours, or say an F1 of the same era, lacks that. So the axle is too thin to drill it, as the hole would punch through the center of the axle. So they make a bracket that attaches to the top, to get around that issue. I saw an F1 done that way not too long ago. Frankly I wasn't impressed. Too much stress on that homebuilt mount in my opinion. Check the classifieds. Ford axles turn up on there from time to time. They are not that expensive, and would be much safer, unless you have a welder with some serious fire power, and lots of heavy steel to fabricate brackets etc. OR.. get a tube axle that would fit your spindles, and the brackets needed to attach the four bars links etc.
Good Luck!