Long before the current 911 RSR caused controversy over abandoning the rear-engined layout, the 911 GT1 took things even further. The GT1 EVO was mostly not a 911, it was an exercise in rule bending designed to compete with the Mclaren F1. The rear half of the car was derived from the Group C dominating 962. The front half bore a superficial resemblance to the 993 and 996 generation 911. Some parts were even pillaged from production models. A forward floor pan from a 993 was utilized. The upper shock-mounts for the 962-derived wishbone front suspension were constructed from 993 strut towers, but that is about where the production-basis ended. A pure 911 the GT1 was not, but it was successful.
Interestingly Porsche featured a road-legal GT1 EVO in this video, reminding us that a few road legal versions were produced for homologation. Indeed, of the 46 GT1s built, more than half were originally road legal. A few racing variants have since been converted for road use, in addition to the original 25 road cars. The roadgoing cars have some differences from the pure racing machines, including tamer MOTEC engine management with proper cold-start parameters rather than the TAG system used in the racers.
These radical cars proved successful in nearly all of their variants. The original GT1, which used the 993 nose, won its class in 1995, only to be bested by the Porsche-powered TWR WSC-95. A privately entered GT1 claimed fifth overall, besting the factory-entered EVOs. For 1998 the heavily-revised GT1-98 claimed the top two spots at Le Mans, and indeed GT1 cars grabbed the top seven overall spots at the event.
Porsche's rule-bending inspired a slew of imitators, including the radical Mercedes-Benz CLK-GTR, Toyota GT-One, Nissan R390, and the bizarre front-engined Panoz Esperante. These rule benders, led by Porsche, priced the class out of existence. Escalating costs effectively ended one of the most vibrant periods in GT racing history.
For those interested, John Brooks published an excellent article on the subject several years ago. It really is worth the read if you care at all about the peak of GT racing.