I'll jump in here...
The engine really doesn't determine the rear end ratio, the transmission and torque the engine puts out does. The 350 is available with HP ratings of 160 HP all the way up to 750 HP and beyond. What type of 350 are you running? You mention a TH350 automatic in a daily driver. A 3.08 ratio would work well as a daily driver, but rather poorly for towing. A 3.73 ratio would work great for towing, but poorly for general economy. The solution could be to go for a ratio somewhere in between say a 3.38 or a 3.55, or use the lower gears and an overdrive transmission like the 700R4. The 700R4 with 3.73 gears will probably give you the best acceleration, performance and fuel economy.
There are other things to consider like peak engine torque and horsepower, and tire size, and of course budget.
As far as breaks go, there are plenty of options out there. 11 and 12 inch disc kits are available in both 5 and 6 lug applications. You may end up getting a kit based solely on matching the rear lug pattern. You might also consider having the rear axles replaced or redrilled with a chevy pattern.
