That rusty line along the frame is definitely the original brake line that ran to the splitter hose, where you are showing the leak above.
It's an annoyance to fish the original broken line out because its clipped on in a few places as I recall so people always leave it. Every ranger I've had has had that main brake line replaced and I have always found the old, original rusty line still there next to the gas tank, plumbed to nothing
FYI, I have not found a replacement speed nut for the bed bolts in a hardware store yet, I assume it's metric. You can use long grade 8 bolts and nuts + washers with tons of anti-sieze to remount the bed. That's what I did since most of my bolts broke and I'd rather have a hex head than a torx bit to deal with in the bed anyway. There are lots of things for a wrench to bottom out on underneath the bed, removing my bed bolts (and nuts) is practically as easy as the stock setup
I'm sure a salvage yard would charge you next to nothing for bed bolts if you wanted to take some
You can usually remove the broken off end of bed bolt from the speed nut at the frame with vise grips and even a plumbers torch or penetrating fluid. The bolts rot away in the middle of their length, get weak, then snap off when you try to apply torque to them
Slather new bolts in never seize on the whole length of bolt, I know that sounds stupid but it really does slow the corrosion down.
Brake line repairs are not as scary as you'd think. I got a flaring tool and practiced on junk sections of 3/16" line and got good at making double flares before I attempted one, but it is definitely not rocket science
I would probably unthread the leaky fitting and inspect the threads to see if they are all cross threaded and messed up. If so you may have to replace the splitter hose- I'm sure all that will be obvious when you undo the fittings to get a look. You will also be able to see the flare on the line and see if it was poorly done. Maybe get piece of 3/16" line with 3/8 SAE fitting for 3/16" brake line at an auto parts store and take a look at the flares, will give you reference of what a proper double flare looks like
If you can't tighten the fitting to stop the leak, I wonder if someone used a metric not SAE fitting and it only feels tight because it's cross threading. Even if it has the wrong fitting etc., you don't have to replace that new line all the way up to the master cylinder to fix your issue. If you are going to work on brake lines definitely buy a small tubing cutter suitable for a 3/16" diameter line. Very important. You can then cut the fresh line coming down from the master off wherever its easy to work on, beyond the rear of the gas tank, and re flare with new fittings and even threaded couplers (Do NOT use compression fittings, many states have laws against it and it's not the best idea anyway) to bypass a bad part of the line