Go down to the welding supply and get enough #2 welding cable to make new battery cables, one for positive and one for negative. If the welding supply doesn't have copper terminals, buy them here and solder them onto the cables.....
Copper Battery Terminals
On the other end, use whatever terminal matches up with the fastener size.....
These are for 1/4" fasteners...
Del City - Item #: 906525
These are for 5/16" fasteners....
Del City - Item #: 906625
These are for 3/8" fasteners...
Del City - Item #: 906855
These are for 1/2" fasteners....
Del City - Item #: 907825
Go here and purchase 2 ground straps......
Amazon.com: Heavy-Duty Braided Ground Strap with End Caps: Electronics
Go inside the car and pick a place on the passenger side floorboard in the area of the transition from the flat floorboard to where it changes from flat and goes up at an angle. Look on the other side of the floorboard (in the engine compartment) to make sure you are not going to be drilling through anything that you don't want a hole in. Drill a 3/8" hole. On the engine compartment side of the hole, use your hand grinder, file, sandpaper or whatever you have available to remove all the paint off the sheet metal. You want it shiny silver, about the size of a half dollar. Your ground strap is 13" long, so choose a firewall location for the hole that will allow you bolt the other end of the ground strap to the frame. Choose a spot on the frame and clean it down to shiny silver, about the size of a half dollar. Drill a hole in the frame. If the frame is box section, you will have to drill the proper size hole to use a self-tapping sheet metal screw. If the frame is open section, you can use a bolt with a nut on the bottom of the frame flange. You will be using this frame hole to mount 2 ground straps, one goes to the firewall to ground the body and the other will go to the motor block to ground the motor, so choose your locations so that the straps will reach. If you can't make that work, buy longer ground straps.
From the inside of the car, run your 3/8" bolt through the hole in the floorboard/firewall, using a large flat washer to prevent pull-through. Attach one end of one of the ground straps to the bolt sticking out on the motor side of the firewall. Use a flat washer and nut and tighten down. Now, on the inside of the car, smush RTV onto the bolt head and washer so that you make it liquid-proof from the inside of the car. Now go to the motor side and smush RTV onto the nut and washer to make that connection liquid-proof. Now, go to the frame connections for the 2 cables, tighten down the connection and smuch RTV all over the connection, making it liquid-proof. Attach the other end of the frame/motor cable to a suitable threaded hole either in the block or the head (block preferred) after removing all paint and corrosion down to shiny silver. Smush RTV all over the connection to make it liquid-proof. Now, you have grounded body to motor and frame, motor to frame and body and frame to motor and body.
Ideally, you would attach the ground cable from the battery at the place on the block or head where you made a ground connection from the frame/body. Use heat shrink tubing on all connections.
Now, purchase a new battery that is the proper size for the battery box and secure it to the car with factory pieces.
Now, if the motor still drags, it ain't the cables that are causing it. Look somewhere else, like timing too far advanced.