hammer each very well then hammer an irwin type socket on,usually does it for me though several snap, captive nut chisels off easily and a flange m8 nut is a good replacement as they self hold once nipped a little for full tightening
I hammered on a 3/8" socket then gently nipped them up before undoing. 6 came off without a problem, two sheared but the captive nuts come off easy with a punch up onto the sheared bolt.
Is this the gearbox cross member you are referring to?
Bit of useless information....
The captive nuts have no thread and special self tapping bolts are used during assembly, forced in with a rattle gun.