Removing the oil cooler

After running my fully reco engine for few days, it developed an oil leak from under the fan housing on the left side. So I
pulled the tin off and, as expected, there is leak from under the oilcooler. I removed the nut on the right side as well as the back one on the left side. Now the front left one is just spinning but not coming off. Being that I can't see it, I am not sure whether the problem is a stripped thread or what. What am I supposed to do now to get the cooler off without dropping the engine out, removing the heads/P&Cs then grinding the nut off?
TIA
Raf

If you are going any further i'd just drop the engine as its more difficult to try to remove the head with the engine in - dont even know if you can get at the top studs with it in?

They are only supposed to be torqued to a pretty lowish amount, like 9ftlbs or something i think. check the manual. Although if it is a doghouse or a late style, the cooler should have the large 8mm studs and nuts which rarely strip - especially on the oil cooler.