I looked up some part numbers on nissanpartsdeal to see what was up with this. The short version is I don't see why they wouldn't fit. I've heard of people doing dash and interior swaps, and I don't remember anyone complaining that the seat belts didn't bolt up afterwards. If you want to be sure, you could pick up a few grey belt parts from a wrecker or a part-out to confirm that they mount up the same before you shell out for fresh ones.
I swapped '95 seat belts into my '93, but they were the same parts, just cleaner (PO was a smoker). For the most part, they're not that tough to work with, but be careful with the belt guide clips halfway up the B pillars. Gotta ease those out gently. And take a picture of the rear belts before taking them out, so you don't mix up the three in the middle.
The long version:
I looked up the same part (left-hand front belt buckle, color code K) for '87, '90, '93, and '95, and found three part numbers. '87 is 86843-03G60. '90 is 86843-12G60, but the listing suggests that the 12G60 replaces the 03G60, suggesting it's just a minor design change from the original. '95 pulled up 86843-61G00, which for some reason isn't listed as a replacement for 12G60. '93 pulled up 12G60 and 61G00, because transition year, but searching with my VIN pulled up 61G00 only.
NPD has an actual photo (for a change!) of 12G60. I compared it to my grey 61G00, and so far as I can compare a low-res picture on a laptop screen to a scuffed-up original IRL and determine which one is which shade of grey, I'm like 90% sure the 12G60 is in fact the brown one.
I don't think they changed the part number just for the color, though, because the blue and red buckles also have early and late variants listed for '93.
So, yeah, as is often the case with part numbers... something changed around '92/'93.
Give it a shot, let us know if it works!