My 65 Impala with OEM SBC has a short water pump that locates the 18" fan only 1/2 blade's width into the fan shroud. I think the OEM fan clutch is centrifical. I've been told the fan is supposed to be only 1/2 in to avoid a wind vortex that reduces airflow. I'm swapping in a GM Ram Jet 350 crate engine with a long pump. I'd like to keep the long pump to locate a fan completely inside the shroud for better safety. I've heard that a fan with a thermal clutch is designed to be closer to the radiator & fully enclosed by the shroud. First, is this true? Second, are there any disadvantages with a thermal clutch? Third, what is the correct diameter of such a fan for the 19 1/4" id shroud? Fourth, how do I find such a fan & clutch? The GM catalogue only offers a 19 1/2" fan for the Ram Jet.