The CV kits have kind of dried up. All the parts for rebuilding the stock rear axles are available at Hall.
There has always been risk in committing the car to part modifications which may not have replacement parts available to them. That never has been a good long term strategy.
There are going to be those that have Royal hissy fits and will lecture us all on our lack of vision. Let them supply the parts now to their better solutions and put up or shut up.
I would SUSPECT though that the CV kits were never in large supplies similar to the power steering kits and essentially have to be put together for each customer as an order comes in? Call SAAC Restorations and ask them about availability.
I know Hall doesn't offer them and we know the Byers situation, not that they ever had them so the only other source might be Pantera Miami or Pantera Performance in Colorado. Other then that, you are going to have to make them yourself.
I wouldn't know where the parts were sourced from but I heard Porsche 930 mentioned previously suggesting that's what you start with and then make adapters for it?
Personally I still have the u-joints and am using Halls modified u-joints with no issues. You can get u-joints from Hall at a reasonable price.
I would say that if the existing axles are ok, leave them alone. "If it ain't broken...don't fix it".