another 360/408 over heating

Those fans are probably pulling more air around the gaps between the shroud and radiator and the bypass holes than they are through the radiator. As otherwise noted, the fans aren't up to task, either. I would realistically think that you are looking at something like this, Zirgo Ultra High Performance Electric Fans ZIRZFU16S , and then making a shroud that seals completely to the radiator and fits the fan tightly. One common recommendation I have seen from members here that have a whole lot more experience than me in going with electric cooling fans is to use an electronic fan controller like the Dakota Digital units. These units control the fan speed to maintain a preset temperature. As you have suspected, one potential trouble area is with the J heads. Some of the J head chambers are really detonation prone, and there doesn't seem to be any specific obvious cause other than factory casting tolerance stack up causing improperly shaped and sized chambers leading to detonation.