Check the NHRA rule book. I think there is a limitation (ET) at which the cell has to be separated from the passenger compartment. Some people drop them down into the spare tire well and use aluminum as the separation. Racecraft and some others sell an aluminum bulkhead that fits into the hatch area to provide the separation. I've got a 2000 rulebook around here somewhere so let me know if you can't find the explicit rules.
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Jeff Chambers
T/S #3
11.97 Seconds/114.5 MPH
Chambers Racing Team