So, I got the new fuel pump installed in the tank today (thanks to everyone who gave advice!)
Preparing to put the tank back in, I changed the softlines immediately before the fuel pump, and discovered this:
DSC_0287.jpg
DSC_0288.jpg
The connection between the hard and the soft line was closed with some hard wax/shrink tubing/whatnot. Under the white hard substance there seems to be plumbers tape wrapped around the place where the connector/adaptor meets the hard line.
As far as I can tell the return line has been blocked. Why would you do that, and what should I expect going forward after removing the blockage?