Two bolts are striped on 1983 280ZX, what size are head bolts to get right size heli coil and how hard is it to install heli coil in block ?

You should take the stripped bolts with you to purchase the new bolts at the auto store at the same time you buy the heli-coil inserts. While I could give you a possible size, (some are shorter, some longer in some engines) you have the exact part right there with you. Go ahead and take a good head bolt with you too.

The inserts are of brittle steel and must be installed carefully, but, this is done frequently for the same reason you need them without any problem.

There is also the option of tapping the damaged threads up to the next size, and using a different size head bolt, but you have to decide which is best based on your experience level and the availability of parts.