It can't be the body since the body is, in death, as it was in life

It is not the same, at least from a biological perspective. It can no longer grow, reproduce, or respond; and of course it decays. The possession of a mind may be one way to define life, but it is not universal one, and definitely not a physicalist one.

All things that don't have mass are not physical

I'm not sure that's necessarily true. Photons, for instance, are usually considered to not have mass. The mind is probably not made of photons, of course, but the idea of a massless physical entity--or at the very least one with mass so small that it hardly affects your mass--doesn't seem illogical.

But I don't know what physicalists claim the mind is made of, and whether that substance is massive enough to validate your argument.