40 years ago they didn't know any better. they handled great for the time.. its now 2014 and there are far better choices out there over 9" drums and slant six bars.
again.. if its a drag car then yes put 6cyl bars in it.. a street car that only sees 45mph arounds town sure you can get away with 6cyl bars. but if you really like driving the car.. take multi hour trips on the highway and stuff like that you will want bigger bars... the drive will be light years more enjoyable..