Q: 1987 300zx wouldn’t start but would fire up, changed the fuel pump now it won’t even fire up, the battery is charged

Failure to crank, that is no starter motor operation, does not implicate the fuel pump circuit in any way. First, the battery should be load tested to determine if it is usable. Batteries can "appear" charged but actually be useless for starting an engine. With a fully charged battery that passes a load test, or a new fully charged battery as required, and the immobilizer (security) system off - if equipped, check to be sure the immobilizer is not activated - if the starter motor does not operate at that point, there are only two possibilities: 1) either there is no power to the starter motor/solenoid, or an excessive voltage drop, hence the entire starter circuit would have to be diagnosed and the fault found and fixed; or 2) there is power but the starter/solenoid itself is bad and that includes intermittent operation which does happen. In scenario No. 2 the starter motor would simply be replaced. If you desire that a specialist from YourMechanic determine which of these is the issue, and get it resolved, please request a no start diagnostic. If additional problems exist apart from the starter circuit failure, the same mechanic would of course deal with that as well. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.