While sizing softeners tends to start a strange debate here everytime. I will post a cheat sheet that takes the guesswork out of it for 90% of the applications. You application slightly exceeds the chart I am posting but it will be close enough.
That being said, I would almost never recommend a 32,000 grain system for pure financial reasons. As an OEM builder of units, I charge less than a $20 difference between a 1 and a 1-1/2 cu. ft softener. You gain a 50% increase in size for less than a 5% increase in price. I sell over 75% of our units we build as 1.5 Cu. Ft. system. The majority are larger. We sell a lot of 9X48 tanks, but usually for other treatment methods and systems.
Now for your system,
300 gallons per day X 14 grains water hardness X 6 days between regenerations = 25200
Your 32,000 grain system is actually a 24000 grain system when regenerated with a proper amount of salt. You should never try to achieve 32,000 grains out of a cu. ft of resin, it would be the equivalent of saying a truck gets 90 miles per gallon, without stating that the test was done with the truck in idle going down a steep hill with an 80 MPH tailwind.
Your system shoud be a 25,200 grain system, round up to the next size = is a 1.5 cu. ft. system.
FYI, my own house is 210 gallons per day, x 13 grains water hardness with a 48,000 actual grain capacity system. It regenerates every 15 to 20 days.
As for the 7000SXT, that is the premeire valve on the market. Ultra high flow rates, great electronic controls, simple service, outdoor rated, bullet proof. I would recommend either 10% crosslink resin or a gac pre tank to protect the resin. See my previous posts for a few ideas.