The 1980 280ZX is the last year of the older style engine with dished pistons. This is from memory.
1975-80 Z-cars
2.8L fuel injection
155 lbs torque at 3400 rpm
135hp at 5200 rpm at the wheels
1981-83 280ZX
2.8L fuel injection
166 lbs torque at 3000 rpm
145hp at 5200 rpm
Those cars weight in at 2900 pounds about the same as the 280Z although they look much bigger. using a 6-cyl is much better for an autocross racer, but an older 240Z with a newer 280ZX engine would be even better due to it being much lighter.
By opening up the intake and exhaust, including headers, you can add about 15hp that brings you up to 170 lbs/ 160hp
Now keep in mind this is real horsepower on a dyno, not "Honda" horsepower where their 160hp Prelude dynos at 136hp