I still can't believe a AMC weighs 525 lbs...
The older designs were overly conservative on casting thicknesses. As I understand it, this 'over doing it' in the casting design was really just a practical way keeping 'useable' block counts high within a casting process that wasn't always perfect in keeping things centered. Since the subsequent machining operations could correctly locate bores and bosses within the ample available material . . .
The result was a lot of unnecessary bulk between the front wheels.