I hate to answer your questions with more questions, but please tell us a little more about what you are looking for. Will this be a self-supported camping trip, a credit card and motel trip, or possibly a SAG supported trip? Are there any parts of the country you would especially like to see, or maybe skip? How many miles a day do you expect to ride? How do you feel about inclement weather? Will this be a solo trip?
Based on your schedule of 45 days, you will probably have to follow a bee-line route and keep the total mileage to near 3,000 miles. While the Adventure Cycling routes are wonderful, you will probably have to make your own route for much of the trip. However, consider using the Western Express once you reach Colorado or Utah; the truth is you'll probably be following it, whether intentionaly or not, by the time you reach Nevada. For other route ideas, search through the journals on crazyguyonabike.com . Other resources include some of the older RAAM (Race Across America) routes, and the routes followed by supported tours including The Big Ride, America By Bike, and PacTour. I think all of these have followed routes close to yours at one time or another.
Leaving in April, be ready for plenty of rain, but you may get lucky. There is a chance of a late season snow storm in the mountains, but it should be unlikely.
Good Luck
Sean