+1 on the Mustang Steve aluminum radiator. It is $199 and a great deal and excellent construction and welds.
I modified my radiator supports to allow me to install his 20" aluminum radiator in place of the stock 17" one.
Here is the write up for the 20 inch conversion for 65-66 Mustangs: http://www.mustangsteve.com/radiators.html
be careful with the electric fans...many won't fit.
the popular one is the taurus fan, but it won't fit "stock" radiator without triming the shroud, or relocating the entire radiator over one way or the other by a couple of inches (otherwise the snout of the fan motor hits the water pump.
I've heard of contour fans being used.
I know there are fans online you can by that are slim....I'm not familiar with them though
If you are trimming the radiator support openings to accept the 20 inch radiator you will be moving the radiator about 1/2 inch forward. This gave us the needed room for the thickness of the radiator and the electric fan.
Ours is a restomod and we are not worried about "keeping it original".
Here is the 16 inch / 2300 cfm electric fan we went with ($99). You will also need a temperature controller and relay to turn it on and off: Chrome Curved Blade Electric Fan
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