The only problem with mounting it down low like that is that it makes it harder for the radiator to "pull" coolant back from the reservoir into the radiator. Makes it easy for pressurized coolant to escape INTO the reservoir but when it cools down it needs to pull some back into the radiator. That's why normally the bottles are mounted closer to the radiator, that is, if it's a true overflow setup as opposed to a pressurized bottle like a lot of newer cars use. If the overflow tank is mounted close, it's much easier for the cooling system to "siphon" coolant back into the radiator as it cools and contracts.