Battery in trunk

Taking 3-4 pounds of cables out and put 8-10 in. A whopping 5-7 pounds if using 1/0 welding cable like I use. Go on a diet! I guess if you are a stock or super stock racer looking for every hundredth, that might be important. My favorite was a car that ran faster et/mph with two batteries in the trunk instead of one.
A street beater. Not a chance you'll ever notice the weight. Throw one of those acme anvils in the trunk ala Wiley Coyote

Run a ground to the frame rail in the rear and another from engine to frame rail up front.

For the OP, take a set of jumper cables and run from the bat neg to the block up front, see if the car starts easier. If it does, part of the issue is your ground path. Hillbilly diagnosing.
Grounds need to be same size as positive cable.