I don't feel so bad about my 7mpg going up into the PA mountains with a 6K load and 9mpg coming back now. Something I've been considering for a new exhaust system, mainly because I'm cheap and I'd like to retain the all stainless steel system, is adapting a 2007+ 2500HD/3500HD 6.0L exhaust system. I was looking at my dad's and I think I'd call it an upgrade above the Walker bus muffler. If you wack it off right after the cats it's a nice dual 2.5" into 3.5" Y-collector going into a GIGANTIC single 3.5" in/out muffler. I'm thinking it might be possible to find one of those systems in the boneyard for a reasonable price. I think it's the 3.5" out that entices me the most. It's also got a nice mellow rumble to it and probably flows pretty decent based on the HP they're able to get out of that LS motor. I couldn't find a good diagram, but you can get the idea from the pictures below. I'm sure there will be some cutting and welding involved so probably not a straight forward swap. In the end, I'll probably end up going with a bus muffler like you one day, but I will keep my eyes out for this option.