Believe it or not you do not need to remove the exhaust crossover pipe to replace the thermostat on your 2002 Chevrolet Venture. It is not easy but it can be done. The lower bolt does not need to be removed all the way, it is slotted. You will however need to loosen it enough to get the housing over the thermostat. The only suggestion I would make if you decide to remove the exhaust crossover would be to warm the exhaust a bit by running the engine. That along with penetrating oil, will generally help with removing the exhaust nuts and bolts.