starting issue.. ground related

ok guys I am having a starting issue with my firebird. i've got maybe 400 miles on this engine install, when I got done I had a 4 gauge wire with eyelets big enough to stick a bolt threw on both ends left over. I left it out (cause I didn't know where it went) and tried to start the car on the initial start up, I turned the key on and the gauges lit up, turned the key farther and it clicked a couple times then I lost power to everything. Unhooked the battery tried again same result. So I hooked the 4 gauge wire up as a ground between the accessory bracket and the ground post on the passenger side. Instant fix car started right up. last night with nothing changed between first start and now, I had the same issue. I know this ground probably has a 'proper' place, can someone tell me where its at?

update, car started. I removed the wire as I cant seem to find a place for it via google. I've got the factory stuff going from the battery to the post, battery to passenger side motor mount, also have the ground strap in good condition on the drivers side between the coil and frame rail. I'm not sure where this wire goes if anywhere...

I found out that the wires on the ground post were rusting a bit so I cleaned them up with a wire brush and there making a much better connection now....