How much horsepower can a stock 5.3 handle?

GM rated stock 5.3s at about 270–320 hp (Gen III/IV). How much more the stock bottom end will take depends on tune, fuel and luck, and we don't publish a single "safe" number. What we can point to: Speedway Motors' 600 hp and 700 hp turbo 5.3 recipes don't list pistons or rods (the 700 hp version adds a cam, LS7 lifters, head studs and timing chain), and BTR sells a "5.3 stock block max effort" turbo cam. Keep the tune conservative, use enough fuel (injectors, pump; E85 helps), and treat anything past stock as a risk to the engine.