The gt 80's I have seen use a type B piston. Be careful when ordering as some suppliers on the web have them named backwards. Look for the one that has the high pin.
They also have a boost port at the intake. With the reed and the Keihin carb you will probably need to go to a larger jet. Zeda sales the windowed piston if you want one and don't want to make it yourself. You will need the one described as a high pin. I don't know if a windowed piston helps with the boost port cyl. you have. I've never tried it. If you are going to try it or any other changes ie. expansion chamber etc. you should do them prior to jetting as they may change the engines requirements.