Dear Tony.
We all feel your pain.
Every one of us went thru the same frustrating moments at this point or another.
I would like to give you couple of advices before you give up.( And believe me, I was on the way to the store to get a case of dynamite more than once).
1. If it doesn’t work for you, walk away. Start over fresh tomorrow, I promise things will look different.
2. Take bmx672 advice, and replace the hoses as well.
3. Get that tool : Amazon.com: Astro Pneumatic 9409A Hose Clamp Pliers: Home Improvement
No clamp is slipping any ware with that.
Good luck bro. just think of the satisfaction you’ll feel once you will complete the job

"3. Get that tool : Amazon.com: Astro Pneumatic 9409A Hose Clamp Pliers: Home Improvement"

Sears has them too. And I agree, don't try to open the clamps without a cable operated tool. The whole Boxster is inundated with these hose clamps, the tool will pay itself back in no time. I wouldn't have been able to replace the AOS without it.

oops, sorry was duplicate mssg

Guys, I just had this (coolant tank replacement) done. Was money well spent. Bought new tank from Suncoast, excellent service & price, had bought all the goodies - hose clamps, spring clamp tool on a cable, coolant, etc - to do the job myself. (BTW, new tanks DO need to be drilled now, is same tank for later models that don't need the hole. ) But--
Was worried I couldn't complete job in a weather window & didn't have good garage space, so took to my indie. They made it look hard, even with a lift, prior experience, all the tools, and an assistant. Spent most of the day there. Looked like pushing out the grommet next to it was key, and this was something I had been reluctant to do myself, afraid I might dislodge some unseen wiring. Another bonus for the pros: my filler tube was about to give up the ghost, had visible cracks developing in flex portion which I couldn't see with everything installed, would have held me up to get another one, they had one in stock so could keep going.
Power to you guys who DIY this one! But I made the right call for me to hand this one off...