I don't have a real clear picture of exactly what you're trying to join, but I did get the idea of an adapter sleeve. Does one end go over (or inside) your 3/4" tube? Does one end have (or require) a spline on it?
Long story short, I went through some of the same design issues on my Locost setup your are doing here with your Alpine. The best advise I got for joining two components where you didn't had a "D" or Double D end or a spline, was two 1/4" bolts at right angles to each other where the edges of the drilled holes were at least 2 diameters apart (1/2" in the case of a 1/4" bolt).
I talked to 3 different steering component companies. The problem with using smooth shafts is that a high quality, all-around, TIG weld is the recommended way to fasten components. However, you can't weld commercial steering U-joints as you will damage the internal bearings, and alter the properties of the metal. Double drilling and bolting was the only method anyone would suggest as an alternative.
I finally went with the method that Wooward uses. They have splines (of several types) that will accept U-joints, and are meant to be fitted into the inside of your tubing and welded in place as follows:
Attachment:
Woodward-Spline-Weld-Detail.jpg
I don't know if this helps or not because I'm not certain I am visualizing your intended setup, but I hope it helps.
Cheers,