Great job on this oldie! These 1/40 Revell kits stand up pretty well even to today’s kit standards. I have been building some of these old kits lately, and here are a couple of suggestions regarding decals:
1. You can remove most of the yellowing by placing the decal sheet in the sun. I tape them to an inside window - facing outwardly - on the sunnier side of my house. A day or two usually does the trick.
2. before using the decals, try coating them with a clear gloss lacquer such as Testors Dullcote or just clear gloss enamel out of a spray can. This will help to prevent the decal