That's fine.

For now, buy some polyquat, to keep algae under control. You can get it locally, or here:
Kem-Tek 311-6 Pool and Spa 60-Percent Concentrated Algaecide, 1 Quart

That's a pretty good price, and if you have Amazon Prime, you can order Tuesday and have it Thursday. You'll probably want three quarts. What you don't use will keep indefinitely.

Also, order a K2006. You will HAVE to have one, to manage a pool with CYA that high. Info in the test kit page, in my signature. Depending on how you decide you want to run the pool, you may need to buy a pH meter, as well, since phenol red isn't reliable when you push chlorine levels as high as you'll need to do, with 400 ppm of CYA. But, we can work all that out later.

Meanwhile put 15 gallons (yes, 12) of plain 6% household bleach in the pool, to 'keep' the pool, till we get another program going.

Check your pH FIRST -- if it's above 8.0 or below 7.0 it has to be fixed BEFORE you add that much chlorine, since you won't get accurate pH readings after. In that case, only add 3 gallons of bleach (but, buy all 15)