I've seen 351Ws installed without shaving the shock towers, required custom headers. A Clevor is easier because the exhaust exits at a downward angle.
What about 351W swap headers for a 64-66 mustang? http://www.themustangshop.com/351swap.cfm http://www.strokerengine.com/Headers351W.html http://www.vancouvermustangs.com/forums/archive/index.php/t-32409.html