Rebuilt engine won't start

We've rebuilt the engine of my 04 Titan. It's all back in now but it won't start. The engine turns over but it's slow. I've put a new battery in, new cables, I've done the IPDM test and it checks out ok. When I turn the key the battery gauge goes down to nothing and it turns over like the battery is dead. The battery is new and fully charged. It does that even with jumper cables on. I've checked all the grounds and can't find any missing. Any suggestions would be great

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