Pontiac Firebird 1967-2002 - how to tell which ls enigine is in my firebird

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i have a 98 trans am and im not sure which engine is in my car..... yes i know its an ls1 but i bought it from a kid who said supposidly it has a engine out of a 04 gto. he didnt have paper work on the engine but for the price that i bought it at and the engine ran strong and clean i figured who cares if he is lying or not so i bought now im trying to order parts for it and im not sure what to buy?

do i buy parts for a 98 ls1 (i thought these were the weird engines out of the group) or do i buy parts for an 04 gto?

Is there a way i can look up the serial number to tell what it is? if so where do i find the serial number? is there anything on the engine that will be a dead give away as to what it is?

Any help would be great thanks

From my understanding the 04 GTO was still an LS1 5.7L, but i am unsure of the other information, just that its a ls1 block with a 5.7L displacement. The 05-06 gto was a ls2.

Does your current engine have the 'center-bolt' valve covers, or the 'perimeter-bolt' valve covers?
1998 F-Body LS1 was perimeter type.

Also check for head casting numbers & let us know what you have.

Does the car still have the EGR & AIR hooked-up?

Need more info & photos if possible.
I'm pretty sure you won't be buying product meant for the 2004 GTO, ...

The most convenient thing to do would be to possibly buy parts for the 1999 F-body, depending on what you need, ...
Then if there are additional changes to the intak, etc., ..., you may be buying aftermarket or GM OE parts for the 2001+ F-body.

It's really a question that can't be answered with a one sentence reply, as there are too many variables, ...