Mostly applies to carburetor/no-ECM guys who still want the nice fans...

https://www.ebay.com/itm/143561646769

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Has an A/C trigger, and it can be set to either 50% or full speed. Can switch the output polarity for different type fans. Can use just about any kind of temp sensor. The thought that went into this little thing is impressive.

(as a reminder, the truly excellent GM-version SPAL fans to use are: 14" C7 Corvette 15-81914 ($224), & 19" CTS-V/Camaro 15-81927 ($282))