Levi Johnston -- I'm Finally Taking Down Palin

Call it the Thrilla in Wasilla -- Levi Johnston's new plot is to hit Sarah Palin where it really hurts ... by crushing her husband on Alaska's legendary Iron Dog snowmobile race course.

Johnston tells TMZ he's decided to enter the next running of the world's longest and toughest snowmobile competition ... with the sole intent of burying his ex father-in-law, Todd Palin.

FYI .. Todd's won the thing 4 times, so Levi's odds are slim to frozen.

Levi says it's been a lifelong dream of his to compete in the race, which sounds like a ton of fun: pairs of riders tag team their way over a 36-hour, 2000 mile race through the Alaskan wilderness.

Palin had to drop out this year after just 100 miles due to mechanical problems ... so maybe Levi sees an opening next year.

Of course, it's easy for Levi to talk smack since the race is still 350 days away.