Is the floorpan of a super beetle same as an ordinary beetle, diregarding the front suspension? I'm planning to lift the body off my super beetle cabriolet and have a donor car's floorpan. Does my donor car have to be a super beetle too? Would a normal chassis support the convertible? Or do I have to add cross bracings on the floor?

As I recall, they are quite different.
You can inspect this easily yourself.
If you can get the two cars side-by-side, that would be priceless. If not, perhaps consider using a digital camera and compare them.
There are far too many items to inspect to list them here.
This will escalate into quite a project quickly if you've never done this.
What level of experience in these things do you have?
Where will you do all of this?

You will need the donor car exactly the same model. Super, convertible or regular. They were all different. Do an internet search As I remember there are a couple of places that make/sell reproduction parts. But it's been a while since I played with bugs.