I went the other way, as it turns out.
If you lived closer, I'd sell you what I pulled off my '03 1500HD, which was a 2-into-2 Flowmaster (3") with a Y-adapter cutting the outlet down to a single tailpipe. Bought the truck with it installed.
It was more 'deep' than 'loud', but since I use the truck for long-distance towing, and I didn't want to listen to the drone for hours on end, I put a stocker back in its place. Lost a little on the 'butt dyno', but it was worth it for the quiet.
As I look online, it appears they don't cost a ton, so it wouldn't make sense to buy it from me in IL (the shipping costs alone would prolly be $30, out to Cali.) You could get a brand new one for about $75, from what I see. Plus I was holding onto it in case I ever wanted to reconnect it.
The 3" exhaust, which is what you see on the 6.0L and 'HD' gas trucks, is actually considered pretty good, for a factory system. As you probably know, it's a true dual down to the muffler, and at 3" diameter, it's sized just about right. GM did a good job with it. The stock-style muffler is really the only major restriction point. Even the exhaust manifolds aren't bad, when you look at the routing, as 'shorty' headers really don't return much in the way of added power (long tubes are the way to go, if you want that.) Their weight, looks, and susceptibility to cracking are the drawbacks with the stock cast iron pieces. Replacements are cheap (from the pick-n-pull).
My truck's exhaust is made of stainless (?), so it's not prone to rust-thru (plus my truck spent the first half of its life in TX, which helps.) I don't think the earlier-gen pickups had the stainless (you mentioned a hole before your muffler.)
There's a place I found that had a catback for the 6.0L for about $700. I think it might have been a Gibson (?), which doesn't have as many good reviews for SQ as, say, the Borlas & MFs. Those were easily $1000 and up.
