You need the proper tool.
https://www.amazon.com/4525-Cable-Type-Flexible-Clamp-Pliers/dp/B000F5JM0O
Btw, you can cut those short hoses, replace them with common hose from an auto parts dealer.
The hose clamps are called spring clamps and can all be accessed from the top. When you put the new tank in, figure out where to position the clamps so that they're easier to access.
Wayne Dempsey did a nice write up with pictures.
https://www.pelicanparts.com/techarticles/Boxster_Tech/33-WATER-Coolant_Tank/33-WATER-Coolant_Tank.htm