I have recently been trying to find out what this engine is in my firebird. I know it's not the original engine I'm just trying to figure out where it could have came from the identification code is 003877 YF. I'm also wondering if my firebird was even originally a 400 or someone just threw this 400 pontiac engine into this firebird.

YF is not a Firebird engine code.

To nail down the year and rough HP rating..need the date casting next to distributor hole. 1 letter followed by 3 numbers. Also...the head codes would help. Middle 2 exhaust ports..2 or 3 numbers or a combination of 2-3 letters and numbers about 1" in length.

There would be a couple other signs that your car might have been a 400 option car.

What year is it? The '67s say so on the Trim Tag.

Otherwise, there are a couple signs such as does it have the 400 only hood with dual scoops? That doesn't make it one, but would be an indicator if it is the original hood.

Just looking at codes in general, YF could be either 389 or 400.

Cowl tag will read 3F if 400 car.

First group has no number. 2-5 are numbered.