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My '50 gmc #150 has a decided list to the drivers side. At first I thought the one front spring was tired. Replaced that. Then it looked like the bumber brackets might be bent to give the illusion of the tilt. Pulled those off. Now its down to the cab. I looked in the books and found measurements down to the 64th of an inch! Well, I put a 4x4 up under the drivers side, loosened the shackle and front mount, and jacked it up a little. Now it looks a little better. How critical are those measurements?
Another item of interest is the steering column is off center in the notch on the cab. All of the mounts line up well with respect to the frame brackets, but the column is definately offset to rub the inside of the cab notch. Brake and Clutch pedals function okay without interference. Should I just shim to what looks good to the eye and move on?

Just a guess, but I would say the cab mount under the floor on the side that is leaning is probably rusting away and is causing your problem. Especially since the steering column is rubbing on one side of the opening. Does you clutch pedal rub on one side of the floorboard opening? If so, I would check the cab mounts very closely.

Could have a slight twist in the frame from being overloaded/overworked over the years.

I looked the mounts over yesterday. There is no rust out, in fact I was able to loosen all the mounts on the one side without any problems. I'm leaning toward the frame twist theory, as this old 3/4 ton was a farm truck with registered gross weight of 11,000#. That would be way over what the factory plate specifies of 5,800#.
How difficult would it be to straighten the frame? That won't be in the cards anytime soon, but it would be nice to have a plan. Anyone been here? :confused:

That's pretty much what I was planning for the short run. While it's parked I'm gonna pull the rear springs and inspect/replace worn parts. I was lucky on this one that the front spring bushings had not worn so bad to damage the eyes. But the bolt was worn down about 3/16."
The back is probably where the real wear will be. I'm keeping my fingers crossed.

There is also supposed to be some rubber bushings. The one on the drivers side might be missing.

Tony there is some bushings in the rear cab mount shackle. I had expected they would have rotted away. But when I pulled off the shackle mount (leaving a healthy shim between the cab and frame) I was surprized to find the bushings seemed okay. One thing does lead to another. I now realize that until the rear springs are examined and repaired, rebushed, pins and bolts, won't really know just how it will really sit.
Adding to the difficulty, I was not able to park it on a slab to have a level frame of reference. So I guess the next step will be to tear into those springs.... :rolleyes:

I am planning on changing the cab mounts and am wondering if the cab should be blocked at its existing height from the floor. If not and I pull the shackles will the cab come crashing down.

That is what I did, about a 3/4" plywood shim is what I used at the back from the cab floor to top of the frame. This may be different on other trucks. At the front mount I also shimmed that between the cab mount reinforcement and the frame bracket where the mount bolts on.
This is still a learning curve for me. Today I changed the other front spring. After that operation was completed and everything was tight, I looked at the truck and found the tilt was worse than before I started even though I had raised the cab somewhat.
Slowly, a dim bulb in my skull became a little brighter. Tomorrow I'm going to use the jack to raise the driver side rear frame to see if things look better. I suspect tired springs back there-one or both, and of course it all interacts. I may try using some blocks to try to make the distance from the springs to the frame the same on both sides and see what that does. I would say in hindsight, start by examining spring hardware and springs, and the distance from axles to the frame front and back, side to side. Then move on to the cab mounts.