Just so happens I'm doing that this weekend. My 261 tube length is about 8 and 1/4 inches. I say about because mine had a 10 degree bend at the middle away from the block and I straightened it to measure for a new one. There is also a slight flare on the end wwhere the dipstick seal seats. My dipstick is 15 and 1/2 inches from the seal to its tip. The full mark is 1 and 11/16 inches from the bottom tip. The overall length of the dipstick from pull handle to tip is 18 and 1/4 inches, but I think that is not important as different handle designs were used over the years.

Drew