Hi Eddy,
As I had the same question one year or so ago, I will try to reply.
If you have more than 2 UJ's, you will need a bearing to hold one part of your column. Keep in mind that the lower flexible part of the Sierra steering column (the one with the rubber piece) which connects onto the rack is considered as a UJ.
I also wanted to use 2 splines shafts, connected with a UJ, and a bearing, but I gave up on that considering the troubles : the total cost, the installation of the bearing, and so on. If you want, Rally Design sells 380mm long splined shafts, which are a bit long to my mind (my extension was 610mm only), but of course, it depends of the angle you'll have between the 2 half-shafts. They also have UJ's.
If you want to do that, for your inspiration, you can see a very neat installation of 2 shafts + central UJ + bearing on JanneE's build thread for example.
http://www.haynes.co.uk/forums/showt...hlight=Janne+E
On my side, I finally decided to do as explained in the book.
The diameter of the lower part of the Sierra steering column you'll cut is 9/16'' = almost 15mm. So the tube you'll need for your extension will need to have a ID of 15mm. My supplier had only 5mm wall, so that is what I used (OD is quite big: 25mm, it is pretty close to U1, about 5mm, but it fits), but I'm pretty sure other builders used 3 or 4mm wall only.
Also, it is better to use a seamless tube.
Here is how it looks:
2013-06-02_17-03-14 par
Voucht71, sur Flickr
2013-06-02_17-03-30 par
Voucht71, sur Flickr
2013-06-02_17-03-21 par
Voucht71, sur Flickr
Hope this will help
