I happen to visit Alaska once or twice a year and am relatively familiar with the state. Driving Alcan is one of my lifetime dreams. You can actually get an official certificate for driving the Alcan. I envy you for taking this trip. That said, it does take a lot of planning. If your sole goal is to get to Anchorage, you might be disappointed - Anchorage is very much like many other US metro areas. What most of us in lower 48 don't realize is that the state of Alaska is about the size of 1/4 of the lower 48!
1. Before you go buy a book called "Alaska Milepost" (not sure about the exact name). It gives you milepost by milepost of Alaskan roads. Many places go by milepost, not by name of a town, since there are miles and miles of roads with literally nothing but wilderness.
2. I personally think perhaps a relatively economical and less stressful way to see Alaska is by cruise: 1) the best scenery in Alaska is tidalwater glacier, which can only be viewed from water. 2) Some of the most beautiful towns in Alaska are only accessible by boat or plane and do not have road accessibility. State capital Juneau, Ketchiken, and Skagiway are a few examples. 3)lodging is more expensive in Ak, compared to lower 48 and understandably so. Alaska tourist inductry only has a few months to make money, not like Hawaii, where tourists can visit any time of the year. 4) Food (included in cruise) is much more expensive in Alaska since much food was aircargo shipped to Alaska, especially fresh fruits and vegetables. 5)driving takes a lot of planning whereas almost everything is covered in a cruise. The longest time I spent in Alaska was 10 days about 8 years ago. We drove mostly, took the ferry, and even took our rental car onto a train in Whittier. As I recall, we camped 4 nights, one night of free hotel, and the other 5 nights at B&B. We flew roundtrip into Anchorage but did not pay airfare. And we still spent about $1500 on rental car, food and lodging, and tourist activities. Nothing luxurious for that price (our 10 day 3 island Hawaii trip cost about the same but at much better ocean view hotels). I figure we could have spent the same $1500 on Alaska cruise. That said, I'm not trying to talk you out of driving. Driving around Alaska is fun (if you know where to go), which I very much enjoyed.
2. Be sure to check road conditions before you go and along the way. May in lower 48 is spring time but up north and in the alpines, it may still be covered by snow/ice and weather could turn bad on you. Road constructions is another consideration, as northern Canada and Alaska only has a few warm months to work on winter-storm damaged roads. All the road repair work is squeezed into a very short time frame.
3. If you are going to drive the 5000 miles, I would also carry an extra tank of gas. Road services might be limited in May and some gas stations may not be open or may not be open at the time you need it.
4. Don't know if you plan to camp. If you do, you better have some wilderness experience (know for example how to deal with bears). Northern BC and Alaksa have bears all over and spring time is when they are most hungry just coming out of winter hybernation. Sleeping in almost daylight could be another problem.
5. If you plan to take the ferry, you may need to reserve and pay for your ferry ride ahead of time, especially if you need to take your car onto ferry. I booked my ferry ride across Prince William Sound 6 weeks ahead of time. By the way, I highly recommend this ferry ride from Valdeez to Whittier. I met people that were stuck in Valdeez because they don't have a ferry reservation. You also need to know the tunnel schedule in Whittier if you take this ferry ride.
6. Be careful driving on the road - chances are high to hit a wild animal on the road, which is very dagerous for all specis involved.