Recently I helped my brother convert his 1990-1991 TBI 305 to a carbed 350 that I had built and had sitting around.

Right before the motor was changed we had bypassed the VATS (due to a damaged column and not wanting to rekey the cylinder). It worked fine.
After I got the new motor in (hooked up 5 grounds on the rear of the block between the two sides, one engine ground from the battery, all 3 or 4 wires to the main lug on the starter and the one to the smaller terminal, power wire to the alternator and the clip on the alternator) the car doesn't do anything.

The fuel pump randomly turns on when you cycle the key back and forth dozens of times along with the starter randomly making a "clank" noise like the battery is dead. The voltmeter gauge in the car barely pulls a reading. The rear view mirror light comes on very dim. Occasionally the interior lights will come on (off turns them on and on turns them off). I am convinced I have a bad ground somewhere but can't pinpoint it.

Is their anything related to the ECM I need to be worried about? I left the original harness alone (just unplugged) and will trim it down after I get it running.

 

Certainly sounds like you are missing something.
Try hot wiring the starter directly and see if it spins over .If it doesn't check both ends of the batt cables ; not uncommon to have bad / dirty / loose connections preventing full power pass

How are you powering up the fuel pump

 

I have charged and changed out the battery. When I connect the positive and negative wiring I don't get a spark and if I do it is very very weak and unnoticeable. The battery works great, I've since taken it out of the car and used it to turn over a motor on my engine stand. I'd want to swear that the issue very well can lay right at the battery terminals except I see it is defiantly connected.

I didn't touch the fuel pump wiring.

 

So like I said ,jump the starter motor and see if it spins.
If it doesn't ,you are missing something on the 2 main battery cables
Maybe put a DVM on them and check the resistance

If the starter spins solidly then your problem is further downstream in the wiring ; possibly a bad ground as you speculate

 

Got it figured out... this car was in a front end collision and the body shop that did the work never cleaned any areas that the chassis grounds went.