Steve, in the past you didn't need as much lens as you do now.
In order to get any image size you need at least 600 mm and the fellow that
sells on the salt shoots with 1000mm.
Generally aperature is not a problem. With the elevation and the reflectance from
the salt, film ISO can be raised a full stop and have great quality.
Digital Nikons seem to give a rather flat contrast image but work just as well
as film cameras. I have used Nikons for 42 years and have had no experience with Canon.
Don't get too serious about the on course stuff. The way the courses are arranged now
you will have a lot of backlight. Enjoy the staging areas, the pits, the rat rods. the race cars and the
people and you will go home with a full basket.
Bring sunscreen. SPF 15 will work if u apply it several times.
FREUD