Will a 86 Chevy truck bed fit 78Chevy truck?

It's unlikely a 1986 Chevy truck bed will fit a 1978 Chevy truck without modifications. While Chevy used similar designs over the years, there were changes in dimensions and mounting points between those model years. The wheelbases, bed lengths, and mounting hardware are likely different enough to create incompatibility. You'd likely need significant modifications to make it work, and it might not be worth the effort.