I haven't done this, although I do have the axle so I know what you're working with. The very late 82 series-style tractors have the 3-turn box, and it's basically the same as a 149 box, but it'll probably have a longer length, which isn't really a problem. 1811, etc. should have it. The worm from the tilt box should fit if the overall outer dimensions are the same. You'd probably have to compare stuff. If you are going to weld a shaft extension on it, you'd really need to face both mating ends on a lathe, and perhaps put an alignment pin in the center to keep both pieces coaxial and hopefully lessen the warping when it is welded together. Please don't cut up a good steering shaft...I'd probably have a shaft with a bad worm and good splines that could be cut to length
There should be two holes in the pitman arm on the LH spindle. I'm pretty sure the inner hole is for PS and the outer one for manual steering to give you more of a mechanical advantage with the manual steering. The way the geometry is on this axle, IIRC, the arm length for the pitman arm is much less towards the travel extremes, which increases the steering force. There's also some other geometry stuff that's contributing to that that I won't go into. Lastly, if the toe in is wrong, it will be hard to steer. Set that correctly (1/8" toe in) and see if it helps.