I see you own a 500e so you really rather not have that fact pushed on you but Yes a GNX was powered by the V6 Turbo and was quicker than the 500e. Not sure where your getting the info on the 500e. Here is a link to Car & Driver 86 Gn . http://d0n3d.tripod.com/intercooled.htm . You can search out the GNX also. Not trying to get into a peeing contest though just stating it would be a more interesting conversion than a SBC IMHO.
"The acceleration performance of the GNX outpaced the factory's power claims: 0-60 mph (97 km/h) took just 4.7 s with a 13.4 s/104 mph (167 km/h) quarter-mile. According to contemporary sources, these numbers made the GNX the fastest production sedan ever built. This claim is somewhat controversial—the car had two doors but its interior volume and structure made it a sedan rather than a coupe, and just 547 examples were built. GNX #001 is currently owned by Buick and sometimes makes appearances at car shows around the US. Although many quicker cars have been built, including a number of quicker modern sedans, its performance was truly impressive for the time. A contemporary Porsche 930 hit (97 km/h) in 5.0 seconds and ran the quarter mile in 13.6 seconds, roughly equivalent to the GNX, which cost much less and could out-accelerate the naturally-aspirated 911 of the day. Ironically, many believe that these performance numbers were the reason the GNX was axed. GM didn't want one of its own cars outperforming its flagship, the Corvette, and pulled the plug on the Buick program. The muscle cars of the 1960s had the power to beat the GNX, but the tires of the time could not transform this into speed, not to mention the numerous techniques employed in the GNX allowed the car to transfer all the power to the ground, such as a ladder bar that ran from the mid-section of the car to the rear axle, so as to increase traction. This is also the reason why a GNX will actually lift the rear end up when the car is about to launch heavily. The GNX never made much of a road-track competitor to cars like the 911, but it could certainly hold its own on a drag strip."