I think webbcraft's estimates are very safe. I don't believe that the factory published recommended towing capacities during those years. If you plan to tow heavy, make sure you have the appropriate trailer brake controller and a trailer with brakes. Also an auxiliary cooler for the 727 is a must. Also watch your rear axle weight rating.
I have towed 7-8k lbs for several thousands of miles (total) with my '75 W200. It towed the loads with great stability and control. I would say that it was actually very similar to towing the same load with my '98 2500 CTD.
My grandfather used his bone stock '77 W200 (360/727) to tow a large 5th wheel travel trailer, and would occasionally tow his 19' inboard boat behind the travel trailer. The truck did a great job doing that. His truck was set up with good trailer wiring, a good brake controller, and factory overload springs.
2001 Dakota Quadcab - 3.9L V6, Dana 60/10.25", ARB/Detroit, 5.38 gears, 37" MTR's
1998 Ram 2500 Club Cab - 5.9L 12 valve, Dana 60/70, 3.55 gears, DTT 47RE, ~355HP/781FT-LB
1975 W200 Club Cab - 440, 727, Dana 60's, 4.10 gears - slow motion CTD swap
1969 Roadrunner - 383, 727, 8.75", 3.21 gears