If you are not looking for a part # and are willing to work for the part, you can take the one that is on the S10 box and cut the arm off of it about one inch out from the splined end that fits the box. Then cut the original pitman arm that you removed from your car in such a way that it can be welded to the new pitman arm splined piece, attempting to copy the original one with the new splined end for your manual box. It is as simple as it seems, I have done it that way on every one I have installed. The Regal could be ordered with manual steering, and dam few were made. However I will bet your parts yard will not have ever seen one, so this is probably your only option, besides the option 2 that Zip correctly pointed out is likely the only other way you can go.
When you get to the manual brakes, you will end up making your own pedal out of the original power brake lever as well, if you want the pedal where your foot wants it. The leverage ratio needs to be changed from power brakes and that is why the custom pedal is the best way to go.
I thought I would throw the brake pedal lever in the mix so you get mentally adapted to making your own parts. :wink: