I am not positive about the 1989, but here is what I can say that may help:
1) I think I have seen that there were some metal tanks available as options back then (such as a single "crossover" tank). Not that that really matters since you are replacing it.
2) I looked at a 1990 Cruiser and I didn't see anything substantially different in the fuel tank area (it had the often standard 18 gallon rectangular Moeller tanks). I believe the new tanks I put in my 22 would have fit in it as well.
3) I have the Moeller tank part numbers in my album - they are the 23-gallon pair (port and starboard mirrored design) that the builder was using as current two summers ago (and likely is using now). I've also mentioned them in a few threads, as Captain's Cat related.
4) Here is the one thing I would check more closely: Where do your fuel fill(s) come through the gunwale in relation to where the filler necks are on the Moeller tanks? This is something that is pretty much "set" on the tanks, and you can only put so much bend into a short fill hose. Of course you could put new fills into the boat if necessary, but that would be less fun if it were a surprise. On my boat (and I think other "later model" 22's, the starboard fill was a nearly straight shot down to the tank, whereas the port side has a dog leg (because the lazarette wall gets in the way of a straighter run). I got the "Michelin Man" type hose which bends easier, but it's still a bit of a pain because that side needs to be filled more slowly. Much more of a jog and it would not work at all, IMO (I put the curve of the fuel nozzle facing the same way as the jog and that helps). In fact, if I were to do it over (or re-do it in future), I would make a small cutout in the wall of the lazarette and re-fiberglass it so that there was a "groove" or "tunnel" for the fuel hose to go through in a straighter shot. Your forward lazarette wall may be different, but something to check.
5) The 23-gallon tanks were originally 25-gallon tanks, but then in around 2006 they were cut down by around an inch (when the cockpit sole molding/liner was added, I believe). So you lose two gallons on each tank, but OTOH I was able to fit quite a few things above the tanks that I would otherwise have had to find a home for elsewhere (trim tab HPU, washdown pump, etc. -- affixed to bottom of lazarette compartment so not touching tanks).
6) And as mentioned, I would check over any holes that went into the cockpit sole (which is the core of the hull) and overdrill/fill them. I mounted my new tanks with no holes into the hull (combination of fiberglass angles/epoxy/footman loops/methacrylate glue).
7) At least on my era boat, the new tanks sit back aft of the splashwell upper molding and so it's possible to fit full height fiberglass cover panels if one so desires (some older tanks projected forward of this line and thus weren't able to accomodate full height covers). That said, I put canvas covers on temporarily so that I could easily get in to adjust the shims as the tanks expanded -- and now I find them so convenient I'm not sure I'm going to mount the solid ones! (Do be sure to allow every last bit of expansion room Moeller calls for; I did and I would not have been happy with any less.)