If you corrupt memory, whether in user context or interrupt context, the whole machine will crash. Are you sure you can't do what you want in userspace?
The FPU context is not saved; even in user context the FPU state probably won't correspond with the current process: you would mess with some user process' FPU state. If you really want to do this, you would have to explicitly save/restore the full FPU state (and avoid context switches). It is generally a bad idea; use fixed point arithmetic first.
Depending on configuration options the kernel stack is about 3K to 6K for most 32-bit architectures: it's about 14K on most 64-bit archs, and often shared with interrupts so you can't use it all. Avoid deep recursion and huge local arrays on the stack (allocate them dynamically instead).
Let's keep it that way. Your code should be 64-bit clean, and endian-independent. You should also minimize CPU specific stuff, e.g. inline assembly should be cleanly encapsulated and minimized to ease porting. Generally it should be restricted to the architecture-dependent part of the kernel tree.