Because the car was ordered that way. With trailer tow package, with heavy-duty package...or just with upgraded brakes and rear axle. The bigger brakes and rear ends were completely available with 6-cylinder engines; your brother-in-law's guess is probably wrong; a V8-to-6 swap is certainly possible on the '67-up cars, but it requires a whole different K-frame. And the transmission would need changing. Other details, too. Check the car's VIN; what's the 5th character? Probably a "C". For that matter, if you accurately post the content of the fender tag (on the driver's side inner fender, a 2" × 3" metal plate stamped with letters and numbers) we can tell you a fair good bit about how, where, and when your car was built.

(Also...your .signature pic is kind of on the big side, eh? We have members here still on slow connections, and a big sig pic like yours slows the works down for them, bigtime. Mind resizing your sig pic so it's no more than about 225px in either direction, resolution of 72dpi? Thanks.)