Yes they will flow if they fail. However depending on which way they fail means if its going through the cooler or not. If it failed closed while towing it could mean disaster and unless you have an oil temp gauge you will never know it.
Greg
You are right adapters are available. But with their cost it just makes more since to go with a new cooler with NPT threads. We use to sell lines only which was over 400 sets with adapters. Then the adapters really went up in price especially for the 5/8 lines. Plus the adapters did not work...
Depending on why the old motor died would depend on if I flushed the old one or bought a new one. Plate heat exchangers are very difficult to clean properly. If the old motr had a major failure I would opt fpr a new cooler.
Greg