Actually, very early in the 65 production run, Mustangs demand was so hot that the only way to get one was by factory order. My parents bought our 66in May of 65 and it was delivered in November. Many of the published options were in short suppy and were left off. The dash pad and carpet were the wrong color for the car's Ivy green interior, the tach in the performance package and the posi diff were not installed due to shortages. The dealer claimed that these diffs were in short supply due to Vietnam War demand.
Disc brakes were considered exotic and as 2+2 GT said, customers were cheap. Drums brakes and poor braking performance were the norm. I well remember several multi-dozen car pile ups, on the freeway, that my father just barely avoided by swerving onto the shoulder.