It won't hurt anything to reverse flush, if you refill the system and bleed it carefully. IIRC, the flush tee is usually installed in the hose to the heater core, but check the instructions on the flush tee package. On the Series II 3800, locate the hose to the heater core from the engine by finding the arrow indicating direction of flow cast into the nipple where the heater hoses connect to the tensioner assembly. (I'll try to find a pic.)
If the gauge is accurate, 183 is a good temperature to run. If you doubt the accuracy of the gauge, buy a 180F thermostat, drill it, and install it. The drilled hole allows air to bleed from the engine to the radiator gooseneck where it can be purged at the bleeder screw. Locate the hole at the high point because air bubbles will rise to the top of the coolant under the flange, and that way you get most of the air out.
New cap is a good idea, too. I think the standard pressure is 15 or 16 psi. Also, check the condition of the hoses for softness or oil damage.
If you haven't already done so, read this techinfo article about how to refill your cooling system before you open it up. http://www.bonnevilleclub.com/forum/...=article&k=100