Every vertical radiator I've ever seen has the coolant enters the radiator at the top, flows down through the core, then exit the bottom tank back to engine. Gravity does the work. If the coolant entered the bottom and went up, the water pump would have to do the work to push it "up hill" through the radiator. You say you have overheating problems; what makes you think it's the rad? Do you have a fan clutch? Is it working properly? Do you have A/C? Does it engage the fan clutch when you turn on A/C? In most cases, the only time the radiator causes overheating is because the core is clogged. Either from dirt/debris on the outside, or clogged core tubes on the inside. Have you made sure the outside is clean? If the core tubes are clogged on the inside to the point it causes overheating, it will usually make "cool spots" on the radiator that you can feel with your hand because coolant isn't flowing through the clogged portions of core tubes.
