Look up basic heat sink theory on the internet. You have a lot of metal there but not nearly enough surface area. Your transistors will create localized hotspots and more than likely overheat. You need surface area to exchange heat to the atmosphere. I haven't made any calculations but I also think that single fan radiator ain't gonna cut it. You may want to look into triple 120mm fan or dual 140mm fan units.
There is always forced air through a box section extrusion like the pro amps used to use.