Foose 17x8 4.5 bs on 67 Dart ?

Cop wheels, at least the OE ones, are 15x7 and have a 4.25" backspace, or a +6 offset. All of the Ansen's are 0 offset, so, you're moving the wheels out by a 1/4". That's true with either the 7" rim or the 8" rim, they both have 0 offset, the difference in backspace is do to the width of the rim, the centerline is in the same place. So take a look at how much clearance you have now and make sure that you can give up a 1/4" of clearance to the fenders and quarters.

Given the sizes of tires that you can run, there's no reason you need to use the 8" rim, a 7" rim is wide enough to fit a 235, so, you could run 7's all the way around to make things simple.

The 17x8's would work, but you still won't be able to run anything wider than a 215 in the front. With a 17" rim that would probably mean a 215/50/17. The problem with that is that the widest rim a 215/50/17 is spec'd for is 7.5". That's not a huge difference, and you could probably find someone that would mount that tire on an 8" rim, but it will look a little stretched.