An 8.25" would probably hold up fine for DD duties with an automatic, no slicks and 350 HP. Larry's 408 powered AHB has had an 8.25" in it for a few years doing DD with 470 HP. I went 8.75" when I ran small blocks though because I couldn't find an 8.25", so there is that.
You should be able to have the factory 4 spring leafs re-arched with a helper to make a five pack. I did this for many years before I went and bought brand new ones. Old ones didn't sag, just wanted new.
I would think just a set of Monroe Sensatrack shocks with be good for a DD. They're not expensive either. I'm running Edelbrock shocks on my car with the ISO delete, but those shocks were discontinued years ago.