Battery in trunk

crackedback said
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.

I did exactly what @crackedback recommended. Ground cable comes off the battery and goes straight down through the trunk floor to the right rear frame rail. That cable can't be more than 14" or so. Then a same size cable from the right rear cyl head to a nut welded to the right front rail. Again, not very long. And I used either 2/0 or 1/0 welding cable (can't recall) for the positive side...of course through a Ford relay to keep it dead except during crank.

With a mini-starter, the avatar's 10.5 CR 408 will crank like crazy if need be although it normally starts almost instantly. I give @crackedback the credit for my setup!