Yep, this is the exact thing that happened to my truck. Same Saginaw PS conversion as well...I wonder why this is common?
This isn't part of your turn radius problem, but: your steering box is too far forward on the frame - most likely, there's a spud shaft running through the crossmember. This was apparently a pretty common way to do things in the 70's and 80's, but it puts undue stress on both the frame horn, and the rear seal of the steering box. This makes leaks from the steering box pretty common. Just something you might want to address at some point. You can mount the box flush with the crossmember (or even recessed into it a bit). This allows you to get rid of the spud shaft, along with the unwanted pressure on the frame and steering box. Of course, this also means cutting out your mounting plates and re-welding...so it's up to you...
As for the steering - this is more easily solved, usually. The way I did it was:
1. Center the truck's WHEELS (not the steering wheel). Your steering wheel will probably be crooked, or at least still not evenly divided left to right.
2. Disconnect the drag link from the pitman arm. The steering wheel and box are now disconnected from the steering.
3. Center the steering - so you have 2 turns to the left and right. Again, steering wheel might be crooked, but make sure the box is centered - the pitman arm should point straight back, close to parallel with the frame horn.
4. At this point, I disconnected the steering shaft from the steering box, centered the steering wheel, and reconnected the shaft to the box.
At this point, the pitman arm, steering box, and steering wheel should be centered - two turns left, two turns right, pitman arm straight back
5. Loosen the clamps on the drag link, and adjust the length of the drag link so that it can re-attach to the pitman arm. Basically, change the length of the drag link by screwing the ends in/out to reconnect it. DO NOT move the pitman arm.
6. Bolt everything back up (Pitman arm, drag link, tie rod clamps, etc.) to restore steering.
7. Now, you should have the wheels straight, and even turns in both directions. At this point you want to "max out" the steering to one side. Then, adjust the steering stops so that the contact just a TOUCH before the box maxes out. This will prevent the box from destroying itself by stopping the motion of the wheels before the box maxes out. You'll adjust the front stop on one side, and the rear on the other.
Then, max out the steering the other way, and adjust the opposite stops.
You can tweak steering wheel alignment afterward by further adjusting the drag link. You'll probably also want to get the alignment checked, but if you don't mess with the tie rod, it may not be necessary.
There's probably an easier way to do it - but this way worked for me! To simplify, it's: Center the steering box, Center the Steering wheel, then adjust the steering drag link to compensate for the new distance between tie rod and pitman arm.
