After 20 years the cables are probally junk.They might look OK but corrode on the inside.Like said bolt the ground to the block.That and heat the bolt as said.I used my 3/8 Impact a lot removing bolts thast were stuck.A good 6 point socket and some heat turn the power on it down and use some good penetrant.NAPA had/has good 1 gauge cables as factorys were light.If the bolt starts to move work it back and forth and keep heating and penetrant.ive also heated them and then melted bees wax into the threads.One reason I use antisieze on almost every bolt/nut.Rocky