Big DogSS: In case you didn't know, a dual master cylinder has separate reservoirs for the front and rear brakes. So in my mind, by running the rear lines on the driver's side, I don't have to run two lines on the cross member.
To Big Dave's comment: on my 64 Nova and my 65 Chevelle SS, I used steel tubing "sticks" from the local auto parts store to run the rear lines and I am not really a fan of the look. it was clean but the connections were unsightly and an area prone to leaks. On my 55 5-window truck, I used SS tubing that was in a coil from SSBC. Maybe because it was SS, but it was very labor intensive to uncoil and straighten. It took time and patience to bend the tubing for my application but it was manageable. Manually flaring the SS tubing was also a chore. So in the end, given the more complex shape of bending the tubing to rout around the X-frame is why I was asking for a source for pre bent lines for my 59 Impala.