I'd just flush it with water. Be sure you drain the block. It will be a pain to do but it is the only way to get all the old coolant and crud out. I use whatever antifreeze the manual recommends. Prestone G05 is probably what it takes.
Obviously not heat exchanger cooled, but then in a fresh water area no one spends the money for heat exchanger systems. And of course, even exchanger systems can have "issues" with the raw-water side, as the government found out: https://idea.library.drexel.edu/islandora/object/idea%3A1347/datastream/OBJ/view
Another bit of advice, they make some very low cost coolant testers. Its usually just some little balls that float that way you know you have a good coolant mix ratio.
This is what I have always done. Flush with tap water [well water, not city water] drain then refill with distilled water, flush out. Refill with the correct antifreeze, distilled water mix. SCA's with a diesel. I have found the best run time with distilled water. CJ
I'm not really getting the difference between well water and municipal water here. In many cases well water can be extremely hard, contain sulfur, iron, etc. I've done it with municipal water, then then run a few gallons of distilled through it after that. Fortunately the block drains weren't bad to get at and loosened easily. I put them back in with some anti seize so that the next guy should be able to get them out, as I no longer own the truck.
Then the antifreeze was cut with distilled water. Bought the pre charged stuff for the 7.3 Powerstroke that I owned so didn't bother with additional SCA. But that can be had from the local NAPA store as NAPAcool. Test strips can also be used to test the SCA content, but it's my understanding that they have a shelf life.
I deal with municipal water with car washes and the crap that is added will eat away hydraulic fittings in sometimes less than a year. That is the only reason I stated that. If you flush it good, it should not be a problem but I 100% would not use it for a mix. Yes SCA's have a shelf life but test kits are only $50 or so. Any attempt is better than nothing but too much will wipe out water pump seals. CJ