Unfortunately Castor is the toughest thing to adjust. Castor on the yerf is built into the lower mount for the front bulkhead. If the mount is twisted or bent out of square with the frame then everything is off. The mount isn't welded all that great so it doesn't take much to knock it out of wack. I had trouble with metal fatigue in the front frame tube causing my mount to give. I use a piece of angle underneath the strengthen the mount.
It takes quite some time and effort to get this right. You might find a better way but for me I found the only way to know dead on was to take inside measurements of the frame front to back, side to side then draw a accurate square rectangle on a sheet of plywood based on the numbers. drew a center line through the pattern to check the lower mount. Set the frame down on the pattern and see how it matches. you got to get the frame square. Ratchet straps, pipe clamps, whatever. like Tom’s suggestions. then you can check the front mount to see if it lines up square using the center line. it may be advantages to cut the mount loose and re weld properly with some bracing if you find it off. the mount should set dead center , square with the frame and with a 6 degree tilt up.
Then check the bulkhead for square. Measure up down and diagonals across the box and A-arm mounting tabs. A large set of dividers makes this easy. Set the dividers for one measurement then check the rest to see if they match.
The whole idea behind this is, Fact: Something somewhere is out of geometry causing the problem. you got to start at the beginning and work your way out to find it. Basically dis-assemble totally the front end and then re-assemble one piece at a time taking measurements along the way till you find the culprit.
It doesn't show up in this pic but there are layout lines and reference marks all over this sheet of plywood to check square and straight.
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