I moved my elecric stove about 8 feet, and need to extend the 50Amp cable (3 conductors + ground, 6 gauge). How do I splice the ends together. Seems the
6 gauge wire is too thick to use wire nuts.

Use a connector called a split-bolt to join the 2 sections together. Has to be inside of a junction box and wrapped, because split bolts are not insulated.