As you may know the number ON a part is not the part number.
But that number can be traced to an application, and then we could confirm if it is the correct one for your 351M
If it is, the only other thing I could think is that the dipstick
tube is not original and too long.
I do wonder if there is a chart somewhere that lists the length from handle to fill and low marks?
Maybe another member that has a 351M/400 is willing to measure so you have a baseline?
I guess a workaround for you might be to get a dipstick we know is too long (like from a van) and you could stamp or file marks where they belong before cutting to length.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.