With Harrison's death coming so close to the election, I think it unlikely that the Whigs will be able to formally nominate a replacement. The result will likely be an 1836-esque scenario, with each state's Whig electors giving their support to Regional heroes. As the most prominent Whig of the day, Clay would probably be considered the
de facto nominee, but Winfield Scott and possibly Tyler and Daniel Webster could potentially receive support as well.
I don't see this effecting Van Buren's prospects much, though he might gain New York and could possibly loose Virginia to Scott or Tyler. Ultimately, I think Clay ends up winning, though likely looses support to competing Whig candidates in areas of the country where he was less popular.
Henry Clay of Kentucky (WHIG)Martin Van Buren of New York (DEM)Winfield Scott of Virginia (WHIG)Daniel Webster of Massachusetts (WHIG)John Tyler of Virginia (WHIG)