When i did my pumpkin ale I added the spices with like 2 minutes to go in the boil. The sample I pulled tasted pretty good. But I agree with jacksonbrown, you might want to add extra to get the flavor. I used a 29oz can of liby's all natural pumkin, maybe go with one of those and a smaller one.
After a week fermentation was done on my pumpkin ale but I really didn't like the taste. So I added two more 29oz cans with equal parts water and 2 tablespoons mallasses. I simmered it all for about 15 minutes, cooled, then added. It tastes a lot better now.
I once read about the dangers of drinking, I have since stopped reading. - Unknown