How would you remove the clamp from the longeron without damaging either (but if one needs to be damaged, preferrably the clamp."
++++++++++++++++++ My favourite goof last year, did it twice. Heat up the clamp close to the offending bond (propane torch or similar) and keep moving the clamp - it will come free well before any damage is done to the wood. Obvious caution about burning your hand. The heat may effect the joint you were making in the first place, so keep an eye on it - maybe clamp it before heating if this is possible. Steve J