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This is what I'd do:

center the steering box at the mid point in its travel and lock it there so it can't drift down on its own.

Pull the drag link off the pitman arm.

Install the pitman arm on the shaft with the punch/timing marks aligned.

See how much change in the drag link is needed to hook to the pitman arm with the wheels straight forward.

From one of the pictures it looks like the drag link is two tie rod ends with an adjusting sleeve in between. That should give a fair amount of adjustment. That is how my Marmon is set up. 

wheels turned extreme left to extreme right =count steering wheel turns then split difference in half. wheels are going straight; your all set. don't concern yourself of the line up marks. your NOT driving it 100 mile a day everyday, don't forget to change PS filter. we are all waiting for primer and paint on cab LOL

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Mecho, May be I am mis-remembering but on my 392's (and others) there is a skip in the spline, and the arm will only go on in one of four positions. So you can't just put it on in any position.

The internal timing marks set where the external marks are. 

If this 392 is retro-fitted to a 1966 truck, It is impossible to know if it was ever set up correctly. The Sheppard 39 not the 392 was the box that was out in the 60's. 

The end of the pitman shaft has a small section that looks like it has been ground out, possibly to remove the "double spline".

If someone modified it to "make it work" and the next guy came along and lined up the external marks, that lead to the turning tighter in one direction then the other.

If the double spline has been removed, then center the gear, center the tires and put the arm on. If the spline is there, you have to decide how you want to fix it. 

 The reason there are timing marks, both internal and external is that the gear is set-up for a specific application, and in the application, if you assemble "in time" it will be factory correct, the only alignment adjustment (from the steering box point of view) is the drag link adjustment.

When you go your own way, and make it work, without then leaving a record, by putting new timing marks, you can end up like Bob has, where assembling to the correct mark leaves you in the wrong place. 

 If Sheppard is contracted to supply a steering gear, they will know the angle of the box on the frame, and time the internal gear so the external pitman arm will be correct, the internal gear will then have a timing mark on it to mesh with the rack gear timing marks and a unique build number will be stamped into the housing for that build.

 I think in Bob's case a 392 gear from some other application was used to replace the 39 that a 60's truck would have or even a manual box.

 The "correct" fix, would be to lock the main gear at mid point, remove the rear cover, pull the pitman shaft back and rotate until the external timing mark was as close to vertical as you can get (this is one reason why the pitman arm can be fitted in 4 places, so you can get the mark exactly where it needs to be, splitting the move of one tooth on the pinion gear into 4ths), and mesh back into the box, grind a new timing mark on the pinion gear, then re assemble. That does not sound like what was done when this box was originally fitted. 

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agree with all of the above .there are exceptions.  have seen the timing marks off by one or two teeth. for the Dm in question ; making a parade / privately owned truck doesn't require rocket science technology  just factor of safety. as with many projects the question can be asked :: "where were the marks BEFORE it was taken apart "? we all know the answer ++ didn't notice . to build a totally original resto = not this one.  I wouldn't want to be the one to say " Bob it has to come back apart = go tell Mrs mower man .!!! 

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Likely the OCD in me, but I despise Kludge on top of Kludges.

The way that one is mounted, by the main housing to the frame, depending on how much room is "behind" it, it may be possible to remove the rear cover, and pull the shaft back through enough to re time. 

You'd have to lock the main gear at the mid point, then re orient the shaft so the mark is in the right place and re-install, for good measure reach in with a die grinder and make a new timing mark.

If you just make it work by grinding off the double spline, and someone comes along and pulls the arm for some reason you back to where you are now. 

 It is like a Cummins where someone pull the accessory drive off, and then re installed without timing it. It will work, but the next poor soul that has to run the overhead will find (after much work and frustration) that the timing marks aren't timed to anything!

Fix it right once and it will be right, for the life of it. Don't and it will be wrong the entire time it is in service.

 Not like he has to get it together to move a load on Monday!  

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