I'm not surprised a bit. Mid '70's compacts and subcompacts were notoriously unbalanced. After the first Energy Crisis hit, wheelbases shrunk precipitously, along with engine displacement, and almost all vehicles were still RWD at the time. One of my buddies had a '76 Mustang at college. By that time, the Mustang was a pitiful, whimpering shell of a formerly great muscle car. It was bad enough that the damn thing swerved and locked up the brakes at relatively low speed, but if you actually tried to push its puny 4-banger over 60 mph, it would start shuddering and shaking like a tabby cat thrown in a cold bath.
Hate to tell you, but the 1974-77 Mustang II shared the same platform as the Pinto. It was a reskinned Pinto. So, it makes sense that the car couldn't stop. The original Mustang was based on the early 1960 Futura platform).
Ford redesigned the 1978 Mustang/Capri body onto the Fox chassis (The Fox was the platform for the Fairmont/Zephyr). IIRC, today's newest Mustang is based on a modified LS platform (shared with the Lincoln LS/JaguarS/Volvo)