I have this 1987 300z na track car for enduros
Engine is fresh rebuild (2 races) runs good but is lacking top end
So the first issue is where is the top end, barely breaks 105 mph.

2nd what is the best way to add track performance

Welcome to NICO Club Shorebet! Makes me think of sherbert, yum, then shores, which shore are you betting on?

No top end? Fuel restriction comes to mind. You got good fuel pressure, injector spray patterns, etc?

Since you'll be on a track, you'll be with a group. Since you're with a group, there will be rules and classifications. Those typically dictate what you can and can't do for modifications. Are you in the bottomless wallet group (AKA Unlimited) or something more specific? I suggest working backwards from all the rules you hope to conform to. We can surely help, we just need to know your limitations (rules).

In stock form the car should do more than 105 given enough space, not sure what kind of space you are finding on your specific straights. Z31's arent popular track machines so i'm afraid you won't find much specific info on tracking on here or anywhere. Enduro's? Lemons? Chump? WRL? There was at least one guy running a Z31 in the chump series. Might be able to locate him online?

I'd start with the basic's of checking the ecu for codes and verifying the both the physical timing of the cams as well as the ignition timing. I've seen many distributor cars break up on top due to bad plug wires, certainly an easy thing to replace and worth inspecting the plugs, cap and rotor button while your at it.

As for adding "track performance" you generally have more to gain in the turns than the straights on a low power car such as this. Sticky tires, sway bars, springs, LSD, chassis stiffening, it just depends on your budget and your allowed rule set.