There's no need to remove and reinstall the thermostat for the flush. The block drain plug is on the RH side about mid-way between front and rear, under the exhaust manifolds. It's a hex nut with a nipple. You should be able to reach it with a socket or crescent wrench.
Once it's loose attach a piece of about 3/8" thin wall flexible tubing to the nipple, route it to a drain pan, and turn it about one to two turns out. It's very easy to drain the coolant on 103s without spilling a drop. The radiator drain is behind the pop-out panel on the RH side of the front valence and also has a nipple for a drain hose.
A coolant change on the 103 is easier and less messy than on most cars.