You can fit '37-48 spindles to the Econoline axle with very little work. I've got them on on a narrowed Econo axle on my '23 T roadster. No machining of the axle kingpin boss is required. The Econo kingpin is smaller in diameter than the early spindles -- .75" vs .8125". I machined new kingpin bushings to fit the early spindles, but with a bore that would finish at .75" after reaming for the Econo kingpin. The Econo axle kingpin boss is about 1/8" shorter than the early Ford axle boss. This is easily addressed by putting a ~ 1/8" thick washer under the thrust bearing. The kingpin inclination is the same, so no correction to camber should be neccessary.

Bob