1 inch vs 1 1/4 tubing in rad

got a question for anyone who might know. i going to buy a new aluminium radiator for my 340 duster and wondering if the 1 1/4 tubes would be the way to go. it makes somewhere between 475-500 horsepower but doesn't like to stay cool in traffic. it has a swap meet pickup georgia fit all alumium in in now but it just doesn't do the job. i have no idea what the specs are on it. just wondering if any one had some personnel experiance with a prob like this. i have a big electric fan on it and a high flow 8 blade mopar water pump on it and have tried washing and flushing the radiator. thanks guys :thumbup: