Yes, there is a big freeze plug back there. It requires removing the flywheel, and possibly the bellhousing to change it. Flushing the system with an aggressive cleaner will sometimes open up holes that were being plugged up by rust and crud on low or "no-pressure" cooling systems. You might try warming the engine up well and dumping a can of Bar's Leaks into the radiator if you don't want to go to the trouble of pulling the transmission, clutch, flywheel, and bellhousing. It will either fix the leak or it won't, and it might save you a bunch of labor on a vehicle that doesn't get worked hard and driven frequently.
Jerry
