Converting 12VDC to 7..9VDC (20A)
1/3 HP sounds pretty high for an auxiliary cooling fan. It would be good to get an actual measurement of the current draw (running and starting). 

The typical approach used in older cars is to have a series resistor (something like 0.6 ohm for BMW and Mercedes cars). That would imply a current more like 10A from the 12VDC. They're generally in a 25A circuit. 

If it's actually drawing 20A running then the current with reduced (say to 9V) voltage will probably be more like 15A so the power in the resistor or diodes would be less than 50W. Since you have a fan to cool them it should not be  problem either way, but the resistor can be allowed to run very hot, whereas you should keep the diodes to well under 100°C case temperature.