Several years ago I was selling 35 to 38 Ford hollow tube type drive shafts to folks who wanted to make their own traditional lake headers. To reduce the cost of shipping, I would cut those drive shafts in half at center, chuck the halves in a lathe, and cut the welds that held the splined ends in place. Then drive the front and back splined ends out of the tubes. I sold these splined ends separately to guy who wanted to make custom drive shafts like Matt in Alameda is doing, which basically accomplishes the same thing as the Dick Spadaro drive shaft kit.
It looks like I have tracked down the remaining stock of Dick Spadaros' kits. There are supposedly 6 and 12 spline available. I need some feedback to find out if there is demand for these. If so, I may be able to purchase some or all. I have not got into talking money yet but the ones last sold by Speedway(now obsolete item) were $329 in their old catalog. You can PM if you prefer or give feedback here.
I'm not sure if Dick made those himself or if he contracted them out to a machine shop. The shaft can be cut out on a lathe and the splines can be cut on a horizontal mill with the aid of a rotating indexable tool holding device. If further hardening of the finished shaft is needed, it can be done by any number of companies that specialize in heat treatment processes. The rear piece can be made from a 10-spline to 6-spline adapter. A thick wall chrome moly tube will work but there are other seamless thick wall tube types that might work just as well. It just needs to be able to withstand the applied torque of a healthy flathead.
It is difficult to find the old tube type drive shafts in good condition these days.
Mention of solid shafts without a center bearing on post 11, It depends on shaft construction and length of shaft. The model A types got away with it because it was part of the pinion and it was short but FoMoCo found later that they whip too much when suspended between pinion and U-joint. I've seen this on a lot of solid shafts and even some hollow ones. The helicopters generally always have segmented shafts with multiple carrier bearings or they have a center damper bearing on long drive systems like for tail rotor drive.