I'll agree the car is a 49K mile survivor. A t-top GNX - that would be a first. There were a few astroroof GNX's, but no T-top versions. Someone transformed a seemingly lower mileage original GN into a partial clone to their taste and/or budget with what parts they could find at the time. It's not bad. Buyer could take it further.
For the non-knowing, seeing a black Regal with GNX badging would not question the presence of t-tops or the Grand National / Intercooled fender badges. They might not know about the expected Stewart Warner gauges expected on an X car.
I expect the wheels are from Classic Industries. I would expect pics of the inside of the wheels to show the offsets and "Made in Japan" if original GNX wheels.
Fender flares, fender vents, and badging were available from many sources. It would be interesting to see a pic of the trunk area? Is there the expected 9-piece grey trunk kit? How about a pic of the SPID label to confirm the SPID has the CC1 option for the t-tops (hatch roof) so you know it's the original car body and not rebodied salvage.
What is interesting is that this car has a real GNX rear suspension. If it did not come from a donor GNX, the suspension was available from a couple of vendors years ago when the tooling was purchased from ASC. That was about $2200 to purchase + installation cost at the time. As such, a proper exhaust is installed to clear the X suspension components.
A SW instrument cluster can be purchased today for about $2400. Install will take some time as the HVAC ducting gets reworked + the sending units install.
Under the hood, I'll agree with the 49K miles. Original date coded alternator case. Oil cooler hoses date coded 2nd quarter of 1987. The owner could have gotten a re-pop of the GNX turbo heat shield and have no concerns about cutting it for the 3" downpipe clearance. A heat shield would also cover the installed stock turbo turbine housing. GNX turbine housing has a contamination trap. The turbo's J-mounting bracket has extensions for the 4 heat shield mounting bolts to clear the contamination trap. Also could have changed the stock turbo Regal Garrett intercooler tag to the GNX label. The repro GNX label would not have the serial # of the intercooler printed (or you could ask for a number to be printed), but you'd have to look close to see that. And you'd have to look close at the intercooler to count how many cooling fins per row to distinguish between the stock turbo regal intercooler, the TTA version, and the GNX version. All have different fin counts per row but the same number of rows. the up-pipe to the throttle body could be cerrokoted. The driver side relays (3) are not relocated. There are kits available to do that so the inner fenders can be cut and ducted to the outside fender louvers to make them functional. The car has cruise control. Power steering pump reservior bracket has not been refinished from its cosmoline coated and still retains the rubber cap.
The VIN and the door sticker mfg date of Sep 1987 match up. The real GNX079 would have a last six of VIN that begins with 445xxx.
Swap out the GNX grille badge with the correct Buick badge and the GNX emblem on the trunk lid with a GN fender badge and you have a pretty cool, t-top GN with a GNX rear suspension and fender flares. A Grand National insert for the dash plaque is available as a re-pro for $20. Clones used to be looked down upon but times are changing. The past several years at barrett-jackson, mecum, RM, etc auctions, lots of clones for '60s & '70s muscle cars to restomods. They don't bring the money that a real car verified through whatever registry does. However, the builds are done with all the correct parts.
It will be interesting to see how this car sells. It's a nice 49K mile t-top GN survivor.
Attached are a few pics of a real X car highlighting what I mentioned, if someone thinking about buying this car sees this thread.