75 is W & 76 is U. These letters should be on the front of your engine block "WU" with the engine unit "66792_".

The WU means your car should be a 400, 4-speed with AIR (not to be confused with A/C. AIR means it was built for California with our wacky emissions laws). This is consistent with a Zone 24 (Los Angeles) and a dealer code 321 for Superior Pontiac in Monterey Park, CA. Funny, this is about 20 minutes from me.