I think Toma A and Swartllk are just surprised by your choices - I was too. With a manual trans that calculates out to 1835 RPM @ 70 mph. In the ballpark of typical overdrive RPMs. Horsepower is a function of both torque and RPM (RPM you will not have), so horsepower increase is not what to look for. It's a torque increase (at whatever RPM you will be operating at) that you want.

You can increase the sharpness of the torque curve (higher peak but narrower power band) by reducing the LSA (lift and duration being equal). This should not be confused with increasing the horsepower by moving the same torque curve to a higher RPM - which is often done by keeping LSA the same and increasing duration. First camshaft will have more torque/power at a given moderate RPM, but power will fall off quicker at high RPM. Seems that you need your power all down low.

That said, David Vizard (the engineer and engine builder/racer - the one who writes books) claims a formula for optimal LSA in a carbureted small block Chevy without power adders like turbo, supercharger, or nitrous - it is dependent on engine size. For a 350 he claims a 108 LSA cam is always optimal. (400 takes a 105 LSA, 383 takes a 106 LSA and 327 takes a 109 LSA) Then he selects intake/exhaust overlap in degrees, which in turn specifies the duration of the camshaft. Note that LSA affects overlap, and thus the duration requirement. So a 240@0.050 cam is way different for a 108LSA than it is for a 112LSA. The 108LSA cam of same duration has 8 degrees more overlap (which is a world of difference to your engine). Also, a tight LSA camshaft raises effective compression ratio (dynamic compression) because the intake valve will close sooner (for same duration) and with 8.5:1 static CR you need all the help you can get to maintain as much of that as possible. One thing that can be done without really adversely affecting performance at any point is to open the valve further using higher ratio rockers. Vizard always seems to recommend taking advantage of that. You can get his book on Google Play and read it if you want to know more.

Vizard has a cam selection chart in his book How to Build Max-Performance Chevy Small Blocks on a Budget. They are all custom spec Lunati camshafts that can be ordered. DV248-08HFL or DV256-08HFL with 1.6 rockers seems to be what you need. How much increase I don't know, but I would suspect you to be able to feel it - especially at lower RPMs. Don't try to compare duration numbers here to your 112 LCA camshaft (apples and oranges).

Anything that will increase volumetric efficiency (get more air into/out of the engine while those valves are open) will help without affecting driveability. This is where headers and intakes help. But the real power increase these days comes with incorporation of good heart shaped chamber aluminum heads from companies like Profiler or AFR.