Re: freeze plugs
Yes, if one is leaking, the rest soon will. They have all been exposed to the same corrosive forces for the same number of years. A prime example of why the maintenance schedule is drain, flush, drain, and refill with new coolant every 2 years.
Your engine has 4 casting core plugs on each side of the block. There is one on the back of the head. Also, my memory is that there is a large one on the back of the block behind the flywheel.
The eight plugs on the sides of the block are easy to replace, but you will have to access most from under the car. There is a special tool for installing core plugs. It is a driver with a swiveling head. Makes it real easy.
If you can find brass (marine grade) core plugs in the correct sizes, use them and never have them rust again.
Obviously, to change the plug in the head requires removing the head, or pulling the engine. The big plug on the rear of the block requires removing the transmission and flywheel.
You don't think your 280ZX is worth $85/hr. to fix? Is it all rusted out, or some similar condition? Very few cars, including new ones, are as dependable, durable, and as easy to work on as a 280ZX.