I've had 3 - 1978 Royales. And bought a 73 GMC last year to fix up. I had no idea of all the changes over the 5 years. Then Barb's 73 came by with no shroud. So I set out to put my extra 78 shroud on her 73. Went over to look at my 73. Wow. Lots of differences. So did the 73 design go to 77? I'm always towing with a 17,000 lbs GVW. With our 403 - 78 Royale. With absolutely no cooling modifications. Not even an extra trans cooler. Never had any cooling problems. In the mountains, or Arizona in August. So could the 77,78 improvements be the reason for my lack of problems in cooling? With the loads I pull. With my enclosed trailer. I'm at 24,000 lbs. Bob Dunahugh