[which z31 engine]

Water cooling was on all 85+ turbos from the factory. It could have been replaced, who knows.

 

The engines are the same from 84 through 3/87. 4/87 through 89 had the W series engines. Here's the differences: http://www.engr.colostate.edu/~radare/wserieschanges.html

 

Personally, I'd just turbo your NA block and keep the 9:1 compression. That's what I did and so have several others without any problems.

[where can i get pistons...]

If you want custom forged pistons, try BRC Performance. Ordered a set about two years ago when I rebuilt my friends engine and they were by far the cheapest I could find. Quite a few Z31 guys have ordered from them and everyone has been happy with the quality. Not sure if their prices are the same but they were about $475 for 6. Turnaround time is about 3 weeks.

[how hard is it to re-time and put a new t-belt on my 300zxt]

I've used a couple different aftermarket belts and they've all had the right marks. It's a pretty straight forward job. I can do it in under 2 hours but I've done several and have a lot of stuff already removed from the front of the engine. Make sure to do the water pump since one only costs about $25 at Advance Auto. I would also highly reccomend replacing the tensioner AND the stud it mounts on. I had the stud break about a week after I did my timing belt so then I got to replace the heads, too. I've heard of others having the tensioner sieze with the same results.