I'm not sure how, but I have achieved this:
>> gpart show ad1
=> 34 976773101 ad1 GPT (465G)
34 976773101 1 freebsd-ufs (465G)
... which is correct, except for the size.
I have a new problem:
>> newfs /dev/ad1p1
newfs: wtfs: 512 bytes at sector 976773100: Input/output error
Robert Huff