So I was wondering if I could fit the radiator from a z32 into a 1989 z31? And if so, what modifications would i have to do to it?

Probably... The real question is, why? do you just need a working radiator? or are you looking for an upgrade. To my knowledge, the z31 turbo radiator is two row while the n/a is one row. It will fit right in and is generally considered good enough up to 400 ish hp.

i want an upgrade but its also gonna be replacing my busted old one

If your radiator is busted up, and you just need a quick replacement, I would still recommend a z31 turbo rad if you can find one. I don't know about the z32 rad's performance or fitment, but I doubt it cools significantly better than a stock z31 rad.

But to answer your questions: you will need to get a z32 radiator and see if it bolts in without modification. If it doesn't, you will have to fabricate some sort of custom mounting bracket. You could easily do this by bending and drilling into some aluminum bar stock.
Or, alternatively, you can pinch your pennies for now and get an aftermarket three row radiator and call it a day.
https://www.jegs.com/i/Champion-Cooling ... 2/10002/-1

If you're running everything completely stock, the stock radiator is plenty for your VG30E to stay at the perfect temperature. The only time I saw my temperature climb at all was in Death Valley.

Or you can just take your current radiator and get it cored unless it is toast. There are aftermarket repalcement radiators available for Z31, as mentioned getting a turbo or heavy duty will get you extra cooling, no reason to try to jam a Z32 in there.