Well the 240z, 260z, and 280z have had a pretty successful motorsports history ever since they came out (Won SCCA championship in its class every year it was out... 1970-1978)... A stock 240z weighs what, 2355lbs. Thats already 500lbs or so less than an S13, maybe 900lbs less than a Z32. I wouldn't worry about the aero that much either if you really are road racing... You will be modifying the car considerably to race any way, plus it has a small cross sectional area... Lastly, top speed was 125mph for the 240z... with something around 130hp SAE NET. The aero cannot be THAT bad for top speeds, at least any top speeds I would imagine one would see on an average road course.