If you measure the diameter of the threaded portion where it enters the block, the
OD of 3/4" NPT is 1.050 inches. You could also buy a 3/4" NPT male filler plug (two bucks at a local plumbing supply). If the plug fits, you know your engine is threaded for 3/4" (apparently some are 1" instead, with an OD of 1.3").
Assuming your block heater is on the top of the engine block, it's possible to skip draining the coolant -- just work fast! Have the new heater ready with teflon tape applied, choose a cold day, and don't run the engine before you start.
After you install the new block heater, fill the "recovery bottle" (aka antifreeze reservoir) to the "Max cold" line, then start the engine while an assistant watches for leaks around the threads. Idle for 5-10 minutes so the thermostat opens and allows coolant to travel through the entire system. When that happens, you'll see the coolant level in the the antifreeze reservoir drop. If you lost more than a half cup or so, then with the engine still running, crack open the thermostat bleed screw slightly and allow trapped air to escape. Close the screw when coolant starts coming out.