What do they mean

The car looks great as it is, I hope you aren't planning on changing it!
SS/J is the NHRA designation for the class it ran, which is Super Stock J class.
The car was run with a manual transmission and if it was run with an automatic, the designation would be SS/JA
The 326 under the SS/J is the number the car was given by the NHRA.
Maybe a good thing to do is to get your hands on a NHRA rule book and it will break down the class the car ran in.
And to add, the 340 wasn't allowed in that year of car let alone a Valiant in Super Stock because the 340 wasn't introduced until the '68 model year in Barracudas and Darts.
It probably had the 340 in it for bracket racing and the numbers on the car weren't removed.
The car would have ran in Super Stock J with a 273 and a 4 speed.
Very cool.
I wonder what it ran back in the day...