The barrel pivot pin is a bitch to get out and I see no reason to remove it unless the barrel has side to side motion. To remove that screw you need an inpact screw driver, the type you hit with a hanner.
The instalation of a new spring is no big deal just get the spring and guides all set and stand the rifle on it's muzzle and push the plug in and insert a tapered punch in the pin hole, and then you can wiggle around the punch to align the pin and insert it in the breach plug. Even with a high power spring, "if' it's been set' will not be more that 40 to 50 lbs pressure on the spring.
Also do a search on "Gamo tune" here on the forum and you will come up with a nice one with pictureshttp://home.comcast.net/~peterdragin/index.html/