I put a Lokar aftermarket dipstick on my 351W and just found myself sitting on the garage floor w/ my
oil pan using what I figured was common sense. Seems to be working out okay, but I'm not sure how scientific my method was.
Basically, I took the
oil pan and set it up on the ground and put 6 quarts of water in it (it's a 6 quart pan). I placed the dipstick into the water until the water level was about halfway between the "add" and "fill" lines. I gouged a small notch on the dipstick at the same level as the top of the
oil pan (this is also the bottom of the block).
Then I went to the front of the rest of the motor and lined the notch up to the bottom of the block.
I measured how "too long" the dipstick was based on where I was going to mount the bracket for the dipstick tube, and I cut that much off the top of the dipstick then re-attached the dipstick handle.
All I know is that I felt really smart when I was done and had a beer.