A few people have asked me about this recently. As you probably read from this thread, KrF is getting out of it and plans to produce only a few more SFDs. I don't know how they'll share that info if you don't buy their kit, but I'll tell you what I know so we're all on the same page.
BTW, I obtained this info by pulling my own steering apart and doing my own research, so hopefully no one thinks I'm providing anything proprietary here.
The company that manufactures the link is Woodward Steering in Wyoming, US. These guys aren't no-names in the industry. You'll need to call them to order the link. Don't be surprised if you talk to a nice lady and she just writes down your order and payment info.
Their online catalog and pricing is outdated, but still is worth reading for some technical info, like how to clock the u-joints, and the proper operating angles.
The assembled steering link is two u-joints and a short length of splined shaft.
You order it in three separate parts:
1x UA201113 (upper u-joint, ~$70 USD) 1x UA201108 (lower u-joint, ~$70 USD) 1x "Custom UADS" (splined shaft, ~$12 USD)
Shipping has been about $15-$20 for me.
Since I don't run a 4" SFD, I need someone with the KrF SFD to confirm the shaft length. I estimate it at being 5.875" (5-7/8", or about 150mm). To confirm the length, measure the distance between the set screws on the joints halves that hold the splined shaft, then add 1" (this represents the additional .5" on each end of the shaft beyond the set-screw groove). Don't measure the total finished length.
When you order the "Custom UADS", provide them the shaft length (not the assembled link length) and specify that they put set screw grooves on both ends 1/2" in (by default, it's only on one and you machine it to your liking).
For a 3" SFD, I'm using a link with a finished length of 5".
One thing to keep in mind is that, because the steering rack gets rotated, it affects the installed length of the link by upwards of 1/4" or more. You'll need to be mindful of the operating angles mentioned in the PDF I linked earlier so that it doesn't bind.
As you can also correlate in the PDF, the upper splined shaft coming off our steering pivot box is 28-spline x .570". The lower spline on the steering rack is 36-spline x .688". I found that the u-joint ends for these splines needed a very light sanding with high grit emory paper to fit, because the machining on the u-joints is very clean. A 1/8" hex bit is used for set screws. I also used medium Loctite on the screws after confirming that the steering wheel is centered up (hint: take it for a test drive first).
If you find that your steering wheel is off center, you need to re-index the link at top, bottom, or both u-joints. Indexing at the top joint will move it ~13° in either direction per spline, and 10° at the bottom. You may need to compound those to get the correct angle; i.e., if you're off 3°, you need to rotate the bottom joint by one spline in one direction, and the top joint by one spline in the opposite direction (13° - 10° = 3°). The direction of rotation depends which way your steering wheel is off center.