It's been a while since I did my brake lines. I ordered a set and had to re-bend both axle lines as they weren't correct. I bought the whole "kit" as I didn't have most of the original lines in most places (including the one you need to replace) to use as a template. Things change over time and more vendors offer kits now (with some actually advertised for a 3B). From what I have seen, most offer a full kit vs. select lines separately. This line is mostly straight and has bends up near the master cylinder and at the back to angle up in the crossmember to attach to the flex line for the rear axle.
What I've learned about bending lines is it's relatively easy. Yes, I had to redo some but that's the learning part. If you buy pre-measured lines (again, my information may be a bit dated), I found them to be a bit stiffer. Dr. Vern (had a great website-now closed) offered some great tips, including getting a spool of line as it was more plyable (easier to bend). I suspect there are premeasured lines now that are more pliable and easier to bend.
If I was in your spot, I'd remove that long line and slide it out of the frame rail. Measure it and see if your local auto parts store has one that's the same length or a bit longer. If not, your next option could be getting the nuts and enough line to make one. Here you can purchase an extra foot or two and experiment on bending and flaring. To do all this, you'd need a bending tool and flaring tool. I made a bending tool early on but purchased one as well (making it much easier in the long run). Most of the time, I'd buy a line longer, cut it (allowing enough for the double flare), slide the nut back on, flare it and then install. BTW, I used this same method for installing all the 1/4" fuel/vacuum lines too.
The other direction is to replace all of them. Age on lines can show up in blockage and deterioration. Then a whole kit, like what scoutpilot linked to, would be easy and smart. The M38 is nearly the same as a CJ-3A so that would most likely match up with the 3B. Some alterations may be needed. But you could also be getting needed flex line replacements with the full kit too.
One thing I would strongly caution against. Most S lines at the steering knuckle, for some reason, are single flared (at the time, EVERY vendor all had the same S line that was single flared). I would not use those. Everything I've read about brakes indicates double flares are standard. After practicing on my own, I got very comfortable making double flares and Dr. Vern's technique on making S lines was brilliant.
My preferred method of attaching helps me from cross threading. I loosen everything that holds the line secure so I can wiggle it. IF it's never been moved, then this might be omitted - meaning the fitment/thread alignment will still be where it needs to be. But if replacing, I loosen. I then fully insert the flare and ensure it is grounded and square before I begin threading the nut. I will turn backwards first and then try to engage the threads. Sometimes it's helpful to wiggle the nut a bit as you're attempting to turn. Once the threading has started, I can then wiggle the nut and line if needed. What you don't want to do is grab a wrench right off and start torquing, since crossing the threads will ruin both the nut and whatever it is your attaching to - ruining both. Then after the threads are good and snug, you can go back and tighten/ secure everything else correctly. Save all the clips and fasteners when you remove your brake line/lines so you'll have what fits your jeep available to reinstall.
Good tips here:
https://cj3b.info/Tech/BrakeLines.html
