Hindsight is 20/20

I have the tank in but have odds and ends to do, because it got too dark

I installed the sending unit took a bunch of time to get it seated correctly.

After reading the replies above, I have a feeling that I did not secure the clamp on the old tank's new sending unit correctly. I am not sure it leaked there but it was not properly installed and would eventually be a problem.
When I cleaned the original tank I figured I would have to replace it sooner or later.
Well, it turned out to be sooner. Still according to plan.
I really appreciate the informative feedback