I already told you how to do this, and you don't need long jumpers
1....multimeter. "Rig" a piece of wire say 20 ft long. You can even "rig" an extension cord for the purpose. Sized does not matter. Clip one end to the battery NEG and run up front hook the wire to your multimeter. Connect the other probe to the engine block. Crank the engine and read the meter. This will tell you voltage drop in the ground circuit
2...Jumper method Take a decent jumper cable and connect to battery ground. Better if you have two people. Clamp other end to a solid clean ground on the car body. Such as the trunk latch, or other "big" fitting. See if this improves cranking
Then go up front and jumper the engine block to the body. Find a good point on the body it will have to be CLEAN Maybe the crossmemeber. Jumper from there to the engine block see if that changes.
A MULTIMETER IS BY FAR THE BEST TOOL IN THE PLACE. LEARN TO USE IT and you can find and solve hundreds of problems.
