Can you help me with me Deli Spanish?

I work in a Deli in New York, and we have a number of Spanish Speaking customers. I can speak semi-fluently, but have never really come across certain terms. Would any native spanish speakers be able to tell me how to say "I would like a Half of a pound", "did you (pl, inf) say two thirds of a pound" and "Roast beef"? If there are any dialect differences, I'd prefer to know how it's said in Northern Spain. If there are not, I'm fine with any translation.