I think there are performance gains to be made in a stock application. when a manufacturer builds an engine they have to take into account things which a motoring enthusiast might have less concern for, like emissions, fuel economy, expected service life and most of all the almighty dollar!
let's face it, in stock form your average 1970's V8 engine isn't built with any kind of performance in mind, they had cubes and that was enough to make them satisfactory for their intended purpose.
These days there are plenty of cam kits available for a variety of vehicles which will provide a modest increase in performance in an otherwise stock application, especially by owners of later model vehicles who wish to retain their factory fuel injection instead of going for a standalone engine management system.
The same thing applies to our old mills too, Even when you order a "stock cam" you will almost always get something better than OEM.
So I'd say the opposite to you, there's really no reason to run a camshaft designed in circa 1968 on a fresh build.