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[Pranav]

I'm willing to bet money the fuel injectors on your "new" engine are just clogged up. Soak the air filter with with starting fluid and see if it will start/idle.

If it runs with the soaked filter, you most likely are dealing with clogged fuel injectors that have sat around too long; you have the "good" ones off your old/broken motor that you can swap on if that's the case.

Ran into this exact same issue the very first time I tried to start up my first LT1; injectors had sat around for years and were clogged up; had to have them cleaned by an injector shop before I could get it to even start up.

[Pranav]

You can also loan-a-tool a fuel pressure gauge from autozone to make sure you actually have fuel pressure, doubt that's the issue but better to check.

[Supercharged111]

I've had very little success with reusing the plugs on donor engines. Makes no damn sense to me but it remains a thing.