I had the problem too. Turns out the original tube had a taper too it that was too long. I can't remember for sure, but it was tapered a bit either by forming it, or grinding it. This was probably done to start the tube in the hole in the block, but the taper was too long and it wasn't tight enough.
I trimmed a bit off the end, and used some RTV and it was fine. Have to be careful with the RTV. Put it on the tube, not the hole, or some could get pushed inside. I think I had to bend the support tab a bit as the tube was a bit shorter than before.
Since then, I switched to an oil pan with a nipple on it for the dipstick tube. Silly me forgot about sealing off the original tube hole in the block until after the new oilpan was on, so I didn't dare thread the hole for fear of filings getting inside. So I found a short bolt and RTV'd it in. No leaks from doing that either.
On the other hand, I do have oil leaks elsewhere........
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