Brilliant that all your hard work is paying off. Vwd.
As for the drive, driving through DV is not the problem here, it"s the drive over the
Mountains that is. That is irrelevant though.
Driving in US is a doddle and 7 hours to do 400+ miles is nothing like 7 hours (or 400 miles) on uk roads.
Set off early, hit some diners along the way and drive into Vegas at night FTW.