You wouldn't choose one year over another based on factory HP. There simply wasn't any. So you build the engine to have some HP. The 78 is basically an updated 77. Not that many changes externally but the interior has a lot of improvements in door trims, seating, seat belts and T-top design. In your case it still sounds like both need everything gone over, the rust is always a worry, moreso when it's not sheetmetal but chassis. If all thing were equal I'd select the 78 with the improvements, updated interior and belts, better T-tops. In good condition some may prefer the 77 for the same reason because they are hard to find in that condition, but unlikely yours would be. Forget what was true to the Bandit, it was a terrible movie and in the end, any black Firebird is seen as one on the streets, regardless of how much money you have spent or how original it is.