First of all, OUCH! A bad heater core sucks, not only for you breathing in antifreeze, but because to replace one is a royal pain in the arse, and it's expensive. I have never heard of a heater core with AC. I have always just known it to be a heater core, period. The only thing that I have ever read or heard about with replacing the heater core on a 3rd gen Max that has to do with the AC is the discharging of the AC in order to have enough room to pull the bad core out, after you take apart the dash. I was going to mention that you would also need to remove the airbag controller since the core is behind the glove box, but you said your Maxima is a '90 so you do not have any airbags. One less thing to have to worry about.
Is this something you are tackling yourself? If so, my hats off to you. This is from what I have heard a 12 hour job, but if you know what you are doing, and are confident you can replace the core correctly, you can save a bundle in labor costs. I have heard of mechanics charging as much as $1,000 to to replace these things. A new core by itself cost around $300 I think.
I'll leave this post here for a couple or few days and see if it gets any kind of response and if not, I will move it to the Nissan On-line Mechanic section and see if anyone there can help!