When sending your radiator in for repair request that the the top tank be removed and a cleaning rod be passed down all the tubes to wash out each pipe. Pipes on the outer core edge can be repaired by a good trades person. Just preasure testing will not tell you that you have a good radiator, you need to remove years of rust scaling to be effective. As a allternative try locating a Thames truck they have a radiator that fits the Blitz and has a extra tubes in the front row and are a very solid unit made into the 60's. Good Luck