You may have contaminated your steering box
I have a new-to-me power steering problem in my 1962 Thunderbird. I recently upgraded the motor and transmission to a 429 cid V-8 and C-6 automatic from a 1968 Thunderbird. I had to use the power steering pump and new hoses from the ’68 Thunderbird.
The problem is it wants to turn left even while the engine is idling and the transmission is in park. My alignment mechanic says the steering box went bad. I do not see how this could happen with just a motor swap. I replaced the new steering fluid with Lucas steering fluid. There were no changes. Do you have any suggestions?
I agree with your mechanic that the problem is most likely in the power steering box. It could be nothing more than a coincidence that the steering box malfunctioned at the same time that you performed an engine and transmission swap. Everything that breaks does so at some point in time and it’s entirely possible that your steering box just happened to break at the same point in time that you installed a new engine and transmission.
Another, perhaps more probable, explanation is that you inadvertently introduced some contaminants into the box when you did the engine swap. You stated that you used the power steering pump and hoses from the 1968 Thunderbird and there may have been dirt or some sort of contamination in the pump or hoses you installed and once you got the car running the contaminants were introduced into the steering box. A thorough cleaning of the box could potentially solve your problem but the only way to do a thorough cleaning is to at least partially disassemble the box.