When I built the 190/5.0 I had a custom copper rad built to fit but had the same problems in mid summer with AC on. etc.
I went with a different size water pump pulley to pick up the flow rate and had a new aluminum radiator built by Fluidyne, which is in Cal. and Mooresville NC (stock car hdqtrs.) . It is far superior to Griffin which Renntech uses and less expensive. It is what is called a serpentine rad so the water passes three times across the core before returning to the block. It is a pure work of art and should be hung on a wall! All for $397 with custom fittings, one which I made from a block of aluminum for the metric temp sensor. The ends are beveled instead of square bent so water doesn't get trapped in the corners. They have 385 employees and ship commercial...20,000 units a week, and are ISO9000 qualified. If you're interested I can give you more info later as it is at my shop.