Tim, here's the set of measurements from a '54 235, suffix F54Z. Passenger car service, but according to the '68 Chevy Master Parts Catalog, the same dipstick PN 3701556 was used for both car and truck service (photo of the parts listing is first).
Dipstick: Overall, 15-1/4" from grommet to tip; grommet to "Full" (A dimension) 13-5/16"; grommet to "Add" (B dimension) 13-13/16", (exactly by the book, and using the key from the Master Parts Catalog for the measurements).
Dipstick tube length: 6-1/2" - 6-9/16" from block to end (not sure depth into the block).