I drilled an additional 4" hole in the end of the factory muffler on my '74 MCI MC-8, 8V71 with turbo, to the left of the existing 4" outlet. I now have (2) 4" outlets. It sounds only a touch louder, but the output is double. I've only taken it on one 200+ mile trip since, with no real hills to speak of, so I haven't really tested it on any mountain passes yet, but I'm very confident it will make a lot of difference. It genuinely feels like there is more power. The exhaust pressure from both are nearly equal, and now double. I'm not afraid to remove it all together as is recommended on this board, but I only have $60.00 invested in the 4" 90's, etc., so if it doesn't work well, out it goes. We'll find out this coming spring.
While this doesn't specifically apply to your post, and I don't know what issues your current muffler has, I only wish to show that there are options, and I would not be afraid to follow the advice of those on this board and remove your old muffler and install a resonator, or (?) for more power, less heat, better performance.
Good luck, Glennman