I have a Flowmaster cat-back exhaust system on my 98 K3500 with the 7.4L. It uses a 50 series muffler and is not that loud.
Years ago I had a local shop install a duel exhaust system on my 98 K1500 using Flo-Pro mufflers. The truck sounded pretty good going down the road but at idle it sounded horrible, like the exhaust was getting sucked back up the pipes instead of coming out of them. I had the system taken off after about three days and the stock one put back on. I later talked to Flo-Pro and they said that the the shop should have never used two seperate mufflers in the installation. At the time, they didn't offer a single muffler that would work with my factory pipe layout.
In both my 98 5.7L and 98 7.4L, I use the AC-Delco Rapidfire plug instead of the factory platinums with the discs because like you mentioned, these tend to come off over time. The Rapidfire's seem to work fine even though they are a single platinum. I think they recommend replacements of these at about 50,000 miles. On the 7.4L I have the plugs gapped at .045 on advice from another forum but I can't remember if there was a difference or not.
I run dino oil in everything. I used to be a big synthetic fan and Mobil 1 in particular but it made my engine noisier, used more oil, and my oil analysis results were not good enough to justify the cost. Went back to a dino oil and all of the above symptoms went away. This was in a Vortec 350 in a 98 chevy truck that I bought new. I still have this truck today and it runs great.