gns dont weight 3400,
race weight is driver and fuel and spare as it was from factory with the gn rims
typical weight of a GN is 3800+ with driver
though lighter wheels and remove spare and lighter driver (under 200) less fuel and you can have a race weight of a gn in the low 3600s
do alum bumper and lightweight rims , f body rad and 3500s isnt too hard
do some fiberglass parts like bumpers and hood and ditch the a/c and you might get down to a 3400 race weight
factory 245 hp ratings were pretty accurate even with gm policy on not advertising more hp than the vette but making more was easy with a chip better fuel and turn the boost up a little
but , 14.3 ET is accepted as the as shipped performance hen new and with driver race weight of 3800#, punch those numbers in any online calcuators and 255 is about is what youll get