['77 280 Takes a lot of Cranking to Start]

Suggest you "t" in a fuel pressure gauge and see what's going on.  A leaky injector or pump check valve will allow gradual loss of fuel rail pressure causing excessive cranking times.

[280ZX Misfire under idle or slight acceleration]

Very unlikely an 02 sensor is responsible for the symptoms you describe nor would replacing it correct them.  Have you put a vacuum gauge on it?  A vacuum leak is a lot more likely cause. 

[280ZX Misfire under idle or slight acceleration]

Replace the plugs with NGK's and MAKE SURE THE GAP IS CORRECT!

[1977 Datsun 280z will not start]

Crank but not start.

 

You need to diagnose if it's a lack of fuel or ignition problem that causes your Z to crank but not start. 

 

Buy a can of starting fluid (ether).  Spray some in the intake (snorkel, PCV hose, brake booster hose).  This is known as "faux" fuel.  If it will start on faux fuel you have a fuel delivery problem.

 

Any flammable aerosol will do, brake cleaner, starting fluid, hairspray,....the propane propellant  as well as the flammable fluid substitutes for gas. 

 

If it will not start on faux fuel, you most likely have an ignition problem.

 

Pull a wire off a spark plug and stick a screwdriver in there (phillips)  See if it will arc to ground while you crank.  If you have no arc at the plug, pull the wire from the coil off the distributor and try that one.

 

If you have no spark, diagnose the ignition problem.

 

All crank, no start problems are diagnosed FIRST to determine if it is a fuel or ignition problem.