David,
Those do not appear to be Corvette numbers. The block casting number on a GM small block is on the top of the block near the bellhousing. If the motor is cruddy it will be hard to see. You may need to use a flashlight to see it. It will be raised numbers, usually six or seven digits.

Scott

Ranger OK lets ask a stupid question the bell housing on the back of the engine that connects to the tranny right?

Hey here are the numbers that I got off the back driver side

OK according to the numbers it is a 302 327 or 350? How do I which one?

According to my book the B258 means the block was cast February 25, 1968.  That date range would make a 3956618 block a 327. If it was a 1969 casting it would be a 350. I doesn't say what  car it was buikt for. I am sure when Joel sees these numbers he will narrow it it down.  Help us out Joel!

Scott

3956618
This block was used for:
'69 302 Camaro Z28 (early '69, 4 bolt main)
'69 350 Corvette 300/350 hp, 4 bolt main
'69-'74 350 Passenger car, Shevelle, Camaro, Nova, Monte Carlo
'69-'76 350   Truck

Are you certain the casting # is not 3955618?
That would make it a large journal 327, used in '68-'69, Passenger, Camaro, Shevelle, and Nova.

If the info I found on the suffix code is correct(YB=    1968    327    220HP    4bbl   Powerglide Camaro), that would make more sense.

Well thats it. It is a 1968 327 220 thanks for the help. I guess do I rebuild the 327 and super it up or just go with a crate 350 with some teeth? it does to seem to have some pep to it now.

David,
That is a personal choice. It depends on what kind of driving you plan to do. If you plan on just enjoyable cruises and not racing why spend tons on build ups. A 327 is one of my all time favorites. If it were my car I would run that engine unti it's tired and needs a rebuild. Then I would go through the engine and maybe play with a cam, heads, carb, and headers. It,s a fine engine to tune.
Best of Luck,
Scott

Hey Apple I double check the engine code and it is a T05141HB not YB what does this tell you?