I do not take a lot of stock in the FAM because its a 1954 document and not one shred of information is actually from 1950. In this case I am sure its correct, but I am not sure how to proceed.
If I am 1/4" off in the guesswork that is measuring things that are at different heights and planes, the rear fenders will not bolt up properly. Is this something you just wait on until the rest of the sheet metal is ready THEN bolt the running boards up to the rear fender and see where to mark for the mounting holes?
How would you guys do it?