www.factoryworks.com
http://www.mcallisterracing.com/racing/page6.html

Keeping in mind, these bodies require lowered shock towers.

For hard packed dirt, it's gonna be trial and error. Start with a street tread and work from there. Strikers MIGHT do it. If you can see a "blue-groove" developing, then they're gonna work. If not, something slightly more aggressive like a Gladiator or a Bow-tie.