Driveability issues on my slant.

RAT ROD AL said

By the way on a new water pump which is better the 6 or 8 blade ? I heard sometimes if you move the coolant too fast it doesn't have time to cool off or cool the engine down, anything to that? And also , a new pump comes with one gasket , does it also need one between the plate and housing?

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I don't think the 6 vs 8 blade pump is an issue with a slant 6.I think your problem with the temps slowly rising in your motor is in the radiator . I have had this problem more than once in my old Valiants and my 71 Chevy C20. It was always the radiator especially in the automatic transmission cars that use it to cool the trany fluid.An oil cooler may help you cool that fluid( if your van is an automatic) before it enters the radiator to be cooled a second time.I'd also flush out the block with a good cooling system cleaner before putting a boiled out or a new radiator in the vehicle.I believe the root of your problems is in the cooling system and the vapor lock issue is being caused by this.