How is everyone measuring and aligning their cabs on the frame? Mine seems to be twisted counter clockwise wise, not a lot but the radiator support is off to the drivers side and the column where it passes through the fire wall is very close to the passenger side. I found the attached information in my shop manual, it’s for a 47-48 style mount, my truck is a 49 and has rear shackle mounts.
I do not own an "Advanced Design" truck but have owned and completely disassembled/assembled trucks of the following model. Out of curiosity have you measured your frame for being square? While being assembled these frames and cabs were placed into jigs to help keep them square and aligned. These were not absolutely precise processes. Some variation was to be expected, tolerated, and compensated for - especially as the jigs got worn from use. After checking the frame, check the cab holes. Measure diagonally across the holes. They should be close to the same number. You might have to slightly slot the mounting holes to get it all to the point it will all bolt together and fit.
When mounting the cab start the bolts but do not tighten them until all have been started. You should be able to flex it to a certain degree and get everything close.
Keep the forum posted as to what you find. Your solution may be the very information someone else needs to work on their project.
I spent a lot of time helping a friend get his ‘49 3100 back on the road. Like you, we started with a rolling chassis. We ultimately were able to get a good cab fit without opening any holes. We left all the hardware very lose to give us the movement we needed.
The first body part we installed was the cab. It took a lot of time and effort to get it located properly and square on the frame. Similar to the picture you posted, we took lots of diagonal measurements to the holes on the frame where key components would mount.
Another key point to check is the forwards and backwards location as well as side-to-side. If the cab is too far forward you will have issues getting the fenders and hood to fit between the cab and the radiator support. We used measurements from the fenders to determine the correct position.
Also spend time checking the radiator support to make sure it is square. If it is twisted or bent that will create issues.
We had the front end completely assembled before we started on the running boards, bed, rear fenders, and splash shields. On his truck we used a porta-power to bend the running board brackets to locate the front edges of the running boards uniformly to the rear of the front fenders. Then we located the bed and rear fenders to get a good fit at the rear of the running boards. That took some finesse to also get a good fit with the splash shields/aprons.
Patience is key! We would only do this work a couple of hours at a session and walk away before we got frustrated.