Hi Everyone,
Just wanted to give a (hopefully final) update.
The softener is now all back together and seemingly working great! The hardness of my incoming water is 200 and the softened water 20!
I put the 15 lbs of gravel in the bottom and then 1.5 cu. ft. of resin, buttoned it up, went through a full backwash cycle and it's working great!
One lesson I learned is to buy new gaskets as part of the replacement materials. When I first put it together it had a small leak where the by-pass valve connects to the adapter assembly (the part that meters the outgoing water). I have one of those assorted O-Ring sets from Harbor Freight but the one from there was too thick. The over all diameter of the o-ring was correct but the diameter of the actual material was a bit too thick.
I ordered a replacement set online. But, I also stopped at Home Depot to see if they had it. If HD didn't have it, then I would get the correct ones from Amazon the next day.
The O-Rings from Home Depot are size #17 (1-1/16" x 7/8" x 3/32"). The ones from online for my Fleck head don't specify the size (part number #13305) but have an overall larger diameter it seems. So, while not the official right size they worked fine for me!
Oh, and don't forget to lube all gaskets with some faucet grease (food grade silicone). As a handyman I of course had some of this.
Thanks again everyone for all of your great input!
Larry.