OBD II didn't happen until 1996 or so. Anything before that was minimal crap that, half the time, you needed to be a NASA scientist to figure out. OBD I might tell you that you have a misfire. OBD II will tell you that you have a misfire and tell you which cylinder it is, along with misfire counts, etc. OBD II data ports have been standardized since the 90's. So, if you need an adapter, you don't have OBD II.
I had a similar issue on my '98 which is a multiport injected engine. I currently have 175,000 miles on it. On my OBD II, it popped up a misfire on 2 of the cylinders. I had done all 8 injectors at about 120,000 because I knew GM had an issue with the OEM's, so in went some better performing Bosch. So, my misfire. Figured, maybe the injectors were going because I had recently done plugs, wires, cap rotor, etc. Replaced all 8 injectors with more new Bosch. Put it all back together (I have a two piece intake, etc.) and my misfire moved to 2 different cylinders. Say what? Did I just put in 2 bad injectors. I tend to be a little OCD on things and numbered the old injectors when they came out. So I pull the mess apart, again. Replace the "misfiring" injectors with 2 known good ones. Guess what? My misfire moved again???? I'm about ready to light a match and end my problems when it occurred to me that my problem could be spark. It shouldn't be because I had all new in there!! What it ended up being is that my distributor gear was soooo worn, it created all kinds of erratic spark issues! That poor gear was just slapping around in there against the cam. I looked in the hole and the cam gear looked good, so in went a new distributor with a metal gear! The original was a plastic/composite/whatever. No more erratic misfire!! No more unburned fuel smell!! No more hiccuping.
If your motor was rebuilt, it's possible they looked at the distributor and, since it looked okay, stuck it back in. But 275K is an awful lot of miles on 2 gears.
Just a thought.
BTW, I'm an ASE MAster Auto and Master Truck with an Undercar Specialist, Truck Equipment Specialist and some School Bus, Body and other stuff thrown in there. Plus a Mobile Hydraulics certification. My company requires this stuff for my job. They even pay for me to take the tests. I never thought much of them. All it meant to me was that I was good at taking tests! I still learn about this stuff every day because the OEM makes new stuff every day. Like how quickly a new F150 will vaporize in a fire because Ferd thought it would be a good idea to make the bodies out of aluminum. I'd share pics, but there really wasn't much to take pictures of.