The cotton core radiator will loose cooling capacity if you use a Toyota Cotton Core as they simply removed fins. A REAL Cotton Core Radiator has bigger fin material with bigger holes and the cooling capacity is not diminished. The metal is what disipates the heat. Having the existing radiator re-cored by a radiator shop and explaining your problem and wat you want done to the radiator shop deals with that issue. I have yet to see an automotive radiator in an industrial application NOT clog up. Also keeping the corners inside the fan shroud clean is imperitive as you loose 30% of your cooling capacity when the corners are clogged. Like I said and Charlie confirmed, air flow is critical as well and overcoming a poor design is tough.
By the way, the newer Nissans are not going to have the same good results as the old ones due to all their changes I am told.