Complex sentence: The XF6F-6s were the fastest version of the Hellcat series with a top speed of 417 mph (671 km/h), but the war ended before this variant could be mass-produced.
Split into simpler sentences: The XF6F-6s were the fastest version of the Hellcat series with a top speed of 417 mph (671 km/h). The war ended before this variant could be mass-produced.