An update for those curious: when I looked at the listing for the 1972 Dodge pickup flexible brake lines it said they were 13.75" long, while the 1973 Fury lines said 15" long. I wasn't sure what to do with that, so I went with the Fury line and ended up with the exact same length I already had - which is to say, not great, still shorter than I'd like, but at least I know it does work. I put on the new passenger side line and all is well! It bled fine and the car stops nice and straight. So, yay! Everyone seemed confident that it was indeed the flexible line, which it clearly was, but I wonder: how did you all know that it wouldn't/shouldn't be a blockage somewhere between the master cylinder and the flexible line?