Ok, car is supposed to be a 90 350 TPI, i removed the Intake Plenum and looked at the casting numbers and found 14081006 which indicates it is a 87-89 350 TPI with cold start injector.

Car does not have the cold start injector on it, the IAT is located underneath the plenum, and it has the serpentine belt system. and there is no MAF sensor on it and i dont know if it did originally because the air connector from throttle body to air cleaner housing was missing when i bought it.

Could this be an 87-89 motor that was swapped into the 90 camaro, or a 90 motor and intake with a 87-89 Plenum.

 

There are many possibilities ranging from like you said an older motor swapped in. It could be the original motor with TPI swapped onto it. The place you looked up the casting numbers could even be incorrect. The best way to know what the story is though is to look at the suffix code. It will be in front of the passenger side head where the head meets the block (the suffix code is stamped onto the block). It may be a little hard to spot but there. It will be a 3 character code that represents what motor and car that motor was originally designed to be in.