Procure new battery cables and two new ground cables. Choose a spot on the firewall and on the frame where you can grind down to bare metal and also a boss on the block where you can screw a bolt into that is ground down to bare metal. Connect one end of one of the ground straps to the firewall, using a bolt and nut that will go through the firewall and that you can get to from the inside of the car to tighten. The other end of that strap and one end of the other strap will go to the frame, where you have ground down to bare metal and drilled for a bolt and nut. The other end of the second strap will go to the engine block and be secured with a bolt into a boss, wherever you find one. You can also use the starter mounting bolt if that's where the negative cable from the battery connects. Just make sure you have bright, shiny metal to connect to everywhere. No paint or other gunk to limit electrons.
This is always the first thing to do with any old car or truck you start on. If you don't get the grounds worked out, you will be chasing your tail with electrical gremlins until you do.
That's my best advice. Been there, done that too many times.
I like to use #4 welding cable from the welding supply store and solder on the copper lugs myself. BULLETPROOF.