There are probably a lot of variables that need answering for someone to really answer that question on an engineering level. But I say 10 3/8" grade 5's should be plenty. Especially if they are on different planes (vertical & horizontal)
Speski OffRoad LTD, Cargo Solutions for Toyota 4x4's http://www.SpeskiOffRoad.com
"80 series, with a fridge"
1976 FJ40, Project with little progress.
2004 Starcraft 10RT