As Cujet said, those old V8s were built very simple and were easy to make power with just bolt-on parts. Better carb, header-back exhaust, and adjust the timing curve, it wasn't uncommon to see a 40-50 hp increase (240 rwhp to 290 rwhp). Even into the 90s with the 2nd gen SBC LT1 engine, they were rather choked from the factory with a tiny camshaft, small log-style manifolds, restrictive intake piping, and a pig rich tune. Just fixing those restrictions and tuning will pickup as much as 50 rwhp. Go further with higher ratio roller rockers, electric water pump, underdrive pulley, and other "freeing up power" mods, you can push into the 300-310+ rwhp range. The most I've seen made by bolt-on only LT1 was 329.7 rwhp on E85. Just a camshaft change makes it easy to get into the 350+ rwhp range, >100 hp from stock and still using the stock throttle body, stock intake manifold, stock heads, stock compression, and stock bottom end.
I expect my LT1 to be in the 320-330 rwhp (~375 crank HP) range with just bolt-on parts.