Slow crank

1...What are specs on battery, cranking amps, amp hours, etc
2...What size cables and HOW EXACTLY is it wired including the ground. How is the ground routed from block to body to battery, etc

3..Take some voltage checks. "Rig." a wire with alligator clips long enough to reach from front to back of the car, this does not have to be large, no 20 or larger.

"Rig" a couple of wires (or get a helper) so you can fire the start relay from the trunk area

Start up front. Clip your meter to engine block and to starter post. What is the voltage? If it's below 12.2 or so, the battery is "down" and needs to be discharged.

Crank engine and read meter while cranking. You want to see at least 10.5, the higher the better. If not...............

Run your clip wire back to the back and clip to NEG battery post. Connect that to meter ground lead, and leave other connected to starter battery terminal. Did it come up any?

If not go to rear, Figure a way (starter button, helper) to crank starter from rear. Stab meter direct into battery posts (tops) and read while cranking. Any better there? What is the reading?

What are you using for a starter. If this is the older "wound field" (original type) round you up a Dakota "mini" starter. They crank better and draw less current

NOTE If you are running the starter cable through a "Ford" type relay and then up to the starter, this type problem is why I don't care for that scheme. THERE IS voltage drop through EVERY SINGLE terminal, connection, and switch and solenoid. It all adds up