I think possibly the reason your brake line is short (single line going to the rear?) is that appears to be correct for factory disc brakes. The reason I'm saying this is because where it stops (or comes up short) is where a proportioning value mounts. Then there would be a short line to the low pressure switch. This said is if one is attempting to be correct for a factory 1969 disc brake dart. Just my thoughts. Nice car by the way