On a 68 Valiant the B rear is going to be pretty wide, same for Darts. Not a deal breaker, but you're going to need some pretty significant backspacing to make it work, on the order of 6" or so for an 8" wide rim. If you're planning on running modern rims, that won't be a problem, a lot of the 17 and 18" rims have that much backspacing (ie, mustang rims). If you want to run stock style rims, you'll have a hard time finding something with that much backspace unless you have it made.
