Done the same job some time back: ~ I think Volvo originally used a thread locking fluid on these plugs and they can be a real pain to remove. If you have a plumbing/hobby gas torch then try some heat- it should break the bond without using a great deal of heat.
A long blunt chisel carefully placed at the right angle on one of the hex sides then a sharp blow also starts them off.
Welding a nut onto the removed plugs is a good idea as you say.
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