I need help identifying this engine. I believe it is a 67 Pontiac 400 but am not 100% sure. Please help me ID this engine. I have a chance to buy it but I just need to know exactly what it is.

10.5 compression small valve 400

I'm not an expert on decoding engines, but judging from the info on the linked page, it seems like a 1967 400, just like bigchief also says:
http://www.pontiacpower.net/numbers/

First, casting code ending with 6133 seems like part of 9786133, which would indicate a 1967 400 or 428.

Second, engine code YF. This code was used on several engines from 1966 to 1974, according to the web page. It was used on 1967 400 though, so even this seems right.

Third, head code 143 decodes as 1967.