Well one of the things I notice, is while agree the flash point is above the heater box, you will notice as you get farther away from the flash point along the upper cowl it gets less and less white, and by the time you reach the wiper motor it turns into a light overspray. Vs the passenger side where it is very bright white (very close to the flash), and even the rubber seal looks grey with a white (flash) stripe running along it.
This appears to be a 68 Z from the pic, is there no such examples of 68's with the cowl having any white overspray?