You are right to check the pressure of your pump and the rated pressure of your pipe. As Reach said the 18GS20 can build 190 PSI max. With 20' static at surface you would see 183 PSI. With a static of 70' max pressure at the surface like on the underground pipe and CSV will be 160 PSI. But the pipe in the well will see 190 PSI.
1 1/4" Sch 80 is rated for 520 PSI. Threading cuts out about half the plastic so it cuts the pressure rating in half as well. But that leaves you with 260 PSI rated pipe and a pump that can only do 190 PSI, which is good. It is rare for the pump to build too much pressure to work with a CSV, but we always like to check to make sure.
Then the actual burst pressure of pipe is 2-5 times the rated pressure, which makes it even better.
As long as the pipe, CSV, and pump are rated for enough pressure as in this case, adding a CSV will solve all your pump system problems.