Sparty - The porcelain on my cadet 3 seemed plenty thick and strong; I would be surprised if yours wasn't.
You might need to get a better tool than the factory nut driver, which is hard to get much torque on.
Putting a drop of oil between the washers & nuts makes it easier and keeps the washer from spinning on the porcelain.
Tighten each side a little bit at a time, keeping the top of the tank close to horizontal (with a level set across the top of the tank, or just eyeball for horizontal.) Stop tightening as soon as the front and rear of porcelain makes contact between tank & bowl; this is the proper amount of tightening and will make the tank and bowl very rigid.
Do not tighten beyond when front & rear just make contact, it will not make the tank any more secure, and will just add needless stress to the porcelain. I hope this helps!