I used a Newcastle clone as a base for a Northern Brown earlier this year. My computer crashed, so I'm going from memory here...
I would up your crystal malt contributions and if you are looking for that "English" flavor consider using some of the English crystal malts. As for me...
I did 1/2 pound of 60L and 1/2 lb of 80L in a 5.5 gallon batch. Personally, I didn't see any added complexity from using the two crystal malts and I believe it just added to conditioning time.
I did 1/2 lb victory and 1/4 special roast, but if you're going with MO, I think the special roast would be unneeded (though I love biscuit flavors personally).
I used no chocolate malt in my PM, and I'd think that you'd want mild chocolate if you want to get into the newcastle ballpark.
So, in other words, if it were me, I'd use at least a pound of darker crystal malt, scratch the chocolate malt and up the pale chocolate to 8 oz. I'd probably use more victory, but that's my taste.
Oh, and what yeast are you using? Jamil suggested on his show to use Notty to get a dry finish, and I think that's a nice guideline for the style.