Will a 1981 Chev tailgate fit onto 1978 truck?

It's unlikely a 1981 Chevy truck tailgate will fit a 1978 Chevy truck without modification. While Chevy trucks shared many parts across years, there were design changes between 1978 and 1981 that might affect the tailgate's mounting points, latch mechanism, and overall dimensions. You'd need to compare the tailgates directly to be certain.