did I mistake?

I don't think you made a mistake at all. As stated above, you can really get a lot out of a 318. I have one in my 1964 Valiant and it runs pretty strong. In the near future I'm going to pull it and build it up even more. I agree with that 7 1/4 rear though. You can probably kiss it goodby pretty soon. The 8 1/4 is a good economical option. They're probably all you will ever need for a street car with up to 400hp. I know of guys that race with them also. Good luck, Mike