Raypak 336A pressure relief valve

[clarity1000]

I just had a pool inspection in connection with the sale of my home. My Raypak heater needs a pressure relief valve installed. I have two questions:
1. How do I get the plastic plug out in order to thread the new PRV. It is a 1/2 inch square indented hole, what tool? see picture
2. The hole size looks to be 1", but the Raypak manual speaks about a 3/4 in threaded valve which is readily available in the hardware stores.
What size do you think is correct?

 

[kadavis]

It is 3/4", you can use a 1/2" socket extension bar to fit the square