$45K sound about right on the value of that Z28. Top price on a nice 69 Z28 today is in the $85-95K range.
When insuring, make sure that you have a stated value policy and ask if a certified appraisal is needed. In many cases, the lower valued cars do not need any appraisal as long as the company will give you to full amount insured if someting happends. But if you own a car that is at the top of the food chain, have it appraised. Years ago, if I had a Camaro that was valued over $25K, you needed a certified appraisal. Since then, many of the insurance companies have raised the stated value limit.
Be very vocal with your insurance company and ask them hard questions. If the car is stolen, or totaled will I get my stated value, etc.
Believe me, these companies are not your friends, they are in business to make money and to avoid paying claims unless they absolutley have to.
The NADA books are close, and in some cases not even in the right zip code. I use comps when I'm on the road doing certifications and appraisals. Have done enough to know the value with a little research on all !st gen cars.