Surge brakes may be applied on a downgrade. If you are on level ground and they are adjusted properly, they will not overheat.
For the time being, put in a master cylinder and probably wheel cylinders on all 4 wheels. If your backing plates are still good, just replace the parts as needed.
Be aware that electric brakes will more than likly require you to have 4 new drums so that may be an expense you haven't realized yet.