I have a 1995 Z28 that only misfires while I'm cruising. It is not noticeable while idling or under load. Here is everything done to the car
280FXI cam, LT4 ported and polished heads, LT4 ported intake manifold, port matched 58mm TB, 1.6 roller rockers, LS7 style lifters, hardened push rods, 42lb/hr injectors, 2800 stall, 3.42 gears, 13 vane pump, RPM trans clutch kit.

Last weekend I replaced Opti spark, fuel filter, both 02 sensors, and spark plugs. The opti spark fixed my original issue of the car misfiring and back firing all the time, but now it still misfires when I'm cruising.

 

Have you had it tuned for the mods? Do you get any codes when you scan it?