Something just doesn't sound right... Maybe it's just me, but in 5 years I have not seen much evidence of success concerning situations such as yours, thou the brags are many, I am afraid the truth speaks a darker story.
There is nothing wrong with being excited sir, but the last few years of my life have slowed me down considerably as I learned this Internet is not as easy as it looks.
I may advise taking your time and collecting on a results-based approach that pays a little money over the years, with the understanding that if it happens sooner, then more is due.
See, ranking in and of itself doesn't mean much. Anybody can have a number one ranking for a never-used search-term (or a seldom-used one), that's no trick to perform. The work involved needs to base its results on actual traffic, the number of visitors your customer's site receives in the end will speak far louder than any astonishing arithmetic concerning PR and keywords is concerned.
So you could, for example, ask for an initial down payment of 5 or 10 percent. Then, if within 30 days (or 60 or whatever), their site has gained noticeable traffic, you might consider sending the next request for money, say another 5 or 10 percent.
In this fashion, you have at least 10 months to do what you promise and a nice margin of error allows you close to 2 years to finish the project, should things not go as planned you can also opt out after the initial payment with only a minimal loss to the customer.