WillH over on defender source (lawyer representing several owners) has dug this up:
http://isearch.nhtsa.gov/files/23894.ztv.html
.
An advisory opinion letter issued by Counsel for the National Highway and Transportation Association.
.
"Under a long-standing line of interpretations of this agency, the substitution of a new body on a used chassis alone does not result in the creation of a "new" motor vehicle subject to the FMVSS, assuming that the vehicle continues to be titled and registered with its original model year. Thus, under the scheme you outline, the 1967-75 Land Rover with a different body and unmodified chassis could be imported without the need to conform it to the FMVSS."
.
WillH's opinion: "It appears to me that US Customs has been executing inspection of vehicles in a manner that is inconsistent and in opposition to the actual guidelines it is supposed to follow. Under the interpretation above, replacement of non-chassis components should not render a vehicle ineligible for import under the 25-year import rule."
.
http://www.defendersource.com/forum/showpost.php?p=547612&postcount=854