Fluidyne makes a 64-66 Mustang 3-row radiator with one inch tubes that has the lower outlet on the drivers side, but otherwise it's a direct replacement for the stock radiator. Try here www.fluidyne.com .
It is actually not a reverse flow, it is just reverse rotation water pump. The hot water still comes out the thermostat housing. Ford just switched water pump inlet sides in 69 I think.