There is a radiator drain pet**** on the bottom of the radiator on the driver's side. That drains the radiator. There are also 2 block drain plugs-one just behind the oil filter housing at a 45* angle, and the other at the same angle just above the starter on the passenger side. I was able to get the driver's side plug out, but was unable to do the passenger side. If you go after the passenger side plug, REMEMBER to disconnect both batteries. I drained the system as best I could then refilled with tap water. Sealed the system up, ran around for approx. 5 miles, and then came back and drained it out. I did that 2 times. During one of those drains, I added a t-fitting in the heater hose just to the passenger side of the fuel filter and housing cover (that's the hose that goes from the top of the water pump over into the firewall). That t-fitting I bought at an auto parts store, and came as part of the Prestone Flush-N-Fill kit. I clamped it on using the 2 hose clamps provided in the kit. After the 2 flushes and refills, I then attached my garden hose to that t, opened the radiator pet**** and driver's side drain plug, and let the hose run. I also started the engine and let it run for a few minutes. After all that, I ran the hose into the degas bottle to try and get some of the dark colored crap stuck in there and then let it drain. After that, I installed a 203* thermostat and aluminum t-stat housing as well as a coolant filter system I purchased from Bob Riley. I installed the filter just under the driver's side seat with valves. I installed it because the PSD block has been known to have SCA's, rust, sand from the forge process, etc. floating around. I installed valves at the in and out ports of the filter housing because the filter unit is below the coolant system, and if you take the filter off without those, the coolant will start to pour out with no way to stop it. If you want pictures I can send you some, but would need your e-mail. A good install page on a variety of topics is at FMTRVT's webpage. I can't seem to install the link here, but if you go to ford-trucks.com and search for his name and then go into his profile, you can find it there.
I then added 3.5 gals. "WeatherPro" I got from a local International dealer. That WeatherPro is the same as the low silicate "Fleetrite" coolant mine needs. After those 3.5 gals., I added 3 8 oz. bottles of DCA4 I bought from the same Int'l dealer, and then completed it by added the necessary amount of DISTILLED water. Distilled because it has much less mineral content. I bought my SCA test strips from Dieselmann, and came in 4 to a package. There is a chart on the back that shows how to read them. I've read that the normal SCA range is 1.3-2.2. I wouldn't bother trying to find out if any additive was put in there since you're going to change the system out anyway. I check the system every couple of days and add distilled water as needed since there will most likely be air trapped in the system until it works it's way out. SCA's need to be checked (I think) around 2 weeks after this process to check the level.
The owner's manual I think says it's an 8 gal. system, but you will be lucky to get 6-7 since there will be around 1 gal. lying resident in the block. After you add the coolant and SCA's, you will only end up putting around 2 gals. of distilled water. That's because of the water/fluids that will never come out of the bottom of the block. Also, after the drains and refills, be careful because the water will be VERY hot. When you drive around, remember to turn the heater up to full temp and full fan. Good luck !
