Is it cheaper to do all the lines at once? Depends. Are you doing it yourself? If so then no, not really. If you're paying a shop to do it, then yes, a little. You are correct that the others MAY be weak, but not necessarily. Best bet is to get a look at them and then decide. Odds are you'll end up replacing both axle lines up to the junction black at the same time. The main line may/may not be as bad. Take a good look at any of the flex hoses while you're there. They're likely to be dry and cracked if not replaced in the last few decades.Just an update for everyone:
After getting the Holley 0-4777C 650 carburetor on, the car ran great. They put it on the dyno, and they were able to get 229 WHP, and 315 lb-ft of torque. This is after they messed with the timing a little bit, and these are the best numbers they were able to get.
However, like a bad dream, on the last dyno pull, the water pump blew, and one of the rear brake lines that was rusted broke. So it looks like I'm going to be putting more money into this.
One question I have is, would it be cheaper to buy all new brake lines and put them in, or just replacing the one rear broken one? My worry is that if that one was rusted, the others are as well.