If you have the FSM for the Z you should be able to look in the trouble shooting section for the tach and the speedo and it will give you pinouts for diagnosing them and those would be the same pins you need to tie into. The connector you want to splice into is the one the comes off the engine harness, there are 2 (one goes to the ecu and one returns to the body harness) the tach, fuel pump, temp sensor, ecu power are all in this connector. The sub harness in the engine bay that goes to oil pressure sensor, alt., speedo, starter, and reverse lights are all in the few connections behind the passenger headlight.
I'm sure you have figured most of this out but just in case you have not there you go... this is all by memory so I could be a bit off but I think that's about right. The auto might be a touch different so you will need to look into that.
For the tach calibration I would use a dakota digital unit that converts the signal before it hits the tach so you don't have to even worry with it. I just adjusted the trim pot on the tach like others have done on this site and that seem to get it close enough for me.
For the speedo I used a ratio adapter that allowed me to adjust the signal through dip switches. This gets a little tricky so you might want to call the manufacture to get it spot on... when you get to that point I can give you a little more direction. If you do a google search for ratio adapter it should come up. I think its called an abbott ratio adapter.
For your accessory issues look into other LT1 setups if memory serves me right there are quite a few ways around the factory setup. I know different models like the trucks etc... might have better positioning of the accessories.
On my setup I designed a bracket to utilize the factory nissan alternator. I don't have any other creature comforts.
Lots of luck to you!
Dan