Hi Guys,
Manny, 240Z were made from very, very late 1969 (but sold as 1970 models?) to 1973 I think. There were maybe some post 1973 240Z carryovers that may have had the 2.6 liter at the beginning of the 1974 260Z production, as there were some very late 1973 240Z that have the hidious 260Z front bumper.
I suspect that you'd be swaping the engine into a 1973 240Z as people who modded 260Z were rare, only one year of production before going to the 1975 280Z and 200 lb curb weight more as I recall.
But having seen some YouTube clips of LS1 240 cars, he is in for one kick in the seat ride, those cars were 2400 lb ish curb weight!! But as I recall from my friends 240Z, speeds above 115 to 120 mph were frightening due to front end wander. They had a bushing replacement for the front end to help with this, and this may also be what the G-nose for the cars was suppose to help lessen, more downforce to keep the front tires planted.
Cheers,
Garry