Replace the whole transmission or just the valve body gasket?

my dads friend told me just to get a transmission and replace the whole thing because it is easier but he thinks my cars problem is the valve body gasket and i have a 2005 ford mustang but i can not find any where to purchase a valve body gasket for this car. If you know drop links below. But the car has shifiting problems. What should i do. (its not the solenoid)

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