No...you do not need to remove the cross-over pipe to change the T-stat. The right side bolt hole on the T-stat housing is slotted so technically you don't have to remove that bolt entirely. It may help to remove the throttle body for better access to the T-stat housing bolts. I've done it without removing the T-body however you'll have to clamp open the spring loaded throttle plate in order to get tool access to the T-stat housing bolts. You'll need some decent wrenches, curved ones are even better. Expect to turn the bolts only 1/4-1/2 turn each time which is a slow process but patience is the key.
Regarding the Dexcool...I no longer use that in my GA and just use regular anti-freeze like many others have done without issues so you don't have to use the Dexcool. Just make sure you flush out the cooling system really well and there are engine block drain bolts on each side of the engine which is the bolt just to the right of the knock sensor "A" in this photo.
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