look on the inside of the drivers door you should have manufacturing date plus engine size, sometimes it can be in the trunk where the spare tire goes, you should be able to tell by the vin example all of the malibus with 350s had a different number in the same place than the ones with 305s or 6 cylinders. The auto part store would know because some of the parts asks for a digit off of your vin to determine if the part fit your car. I would guess your 79 malibu has a chevy 350 and even if its not the original motor someone would have been a fool to not put a 350 or bigger in it. If its not the original motor dont be suprised if it comes back to be the vin of a 6 cylinder car because anyone who know cars knows horsepower and you just wont find it in the 6cylinder a v8 swap is best expecially for a 79 malibu. That is a good car i have always wanted one.
