I did a search in Mapquest for "5000 Longmont #8 Houston TX 77027". It returns a hit for "5000 Longmont, Houston TX 77056-2419". On an anti-Bush website I came across, "5000 Longmont #8, Houston, TX 77027" is listed as a "documented residence" of George W. Bush during that time period.
Longmont is in the Galleria area, FYI.
BTW, 5000 Longmont #7 is currently listed for sale and the description mentions that it is the former residence of Gene Tierney.
See if you can get a FReeper to check the The Texas State Library and Archives. They'll have City Directries for Houston for that time period...
The zip appears legit. The document from Bush's records appears to verify the address. I thought that P.O. Box 34567 looked funny, too, but it looks okay, too.
http://users.cis.net/coldfeet/doc2.gif