in D&D3.5, how do Magic Jar and Alter Self interact?

In D&D3.5, how do Magic Jar and Alter Self interact?

Suppose I am a normal human wizard, and I cast Magic Jar, then possess a huge zombie. Now I want to cast Alter Self. What are my options for picking a type? Which body changes?

Alter Self says, in relevant part:

You assume the form of a creature of the same type as your normal form. The new form must be within one size category of your normal size. The maximum HD of an assumed form is equal to your caster level, to a maximum of 5 HD at 5th level. You can change into a member of your own kind or even into yourself.

I’m ideally looking for a Rules-As-Written answer here, but any well-argued answer is good too.