Nobody replied, so I thought I'd answer my own question. It might help someone else. I changed my coolant today. I removed the plastic splash pan (like you need to do to change the oil filter). I got a better look at the radiator, it DOES NOT have a drain plug.
Here's what I did. From underneath, I used pliers to slide the clamp back on the lower radiator hose. From the top side (while standing), I reached down and pulled off the hose and let the coolant drain. The engine was warm...I started it and let it idle for about a minute hoping more coolant would come out. A little did, but more came out after I stopped the engine. I didn't want to bother taking the recovery bottle out, so I plugged the top vent hole with my finger and blew into the neck until it was empty. It just flowed into the radiator and out the open bottom. I put the hose and clamp back on. It took exactly one gallon of distilled water to fill the radiator. I drove it for 8 miles with the heat on, then repeated the procedure and added 22 oz. of Prestone radiator flush and distilled water to top it off. This time I drove about 12 miles to pick up my son. When we got home, I drained and filled again with a gallon of distilled water. I repeated this until I used six gallons of water and it was coming out clean enough to drink (or make instant coffee). The last time I drained it, I added one gallon of Prestone extended life 5 year/150,000 mile antifreeze. This is the newer green stuff that says it will mix with anything. I know the old original green Prestone couldn't be mixed with the orange "Dex-Cool" stuff. The coolant capacity is listed as 7-3/4 quarts, so the gallon of Prestone should give me a better than 50/50 mix considering what was remaining in the system was darn near pure distilled water. I'll check it with my tester to make sure the freezing point is -34 F or lower. The original 3 year old coolant with 38k didn't look very dirty, but it's nice knowing I have clean fresh stuff in there now.
I like clean fresh fluids! I even use a turkey baster to get out the brake fluid, clutch fluid, and power steering fluid so I can fill the reservoir with new clean stuff. Clean fluids, oil changes, and lubrication...the key to long vehicle life. My '91 4 cyl 5 speed Nissan truck is still running great after 14 years and 334k+ miles. Original engine AND clutch. Never needs oil added between changes, stays on the full mark. All suspension parts (except shocks) are original. Put grease fittings in all 4 ball joints, used a grease needle on the other joints. Never needed a front end alignment, still tracks straight down the road with even tire wear. These Nissans will keep going like the Energizer bunny.