Dipstick tube on my '68 340 is 6-1/2" in length from the boss on the block to its tip. I needed three hands to get a picture, so there's none coming.
Dipstick is 15-5/16" from the flare that bottoms it in the tube to the full mark.
You can measure your tube and adjust your mark higher or lower depending on its length.
Another always accurate way if you know your pan capacity is to change your oil and filter and only put in 4 qts. Run the engine for a few seconds and wait for the oil to return to the pan. Mark your dipstick for 1 qt low. Add a quart, repeat the run and drain down cycle, and make your full mark. Level ground required of course.
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