Well, if you're not wanting to spin higher rpm (billet rod and flywheel), and you wan't quietish exhaust, then your only option is to run a bone stock motor with a good intake and carburetor rejet. You could increase the compression, but you're making a bigger bang in the combustion chamber, and you'll need a good rod to take the abuse.
With the parameters you've set, you're not going to get most of the way to a 301, not even close. Even if you make more power, and you will, it won't have the torque of a 301, which what you're really after.
It's like you're asking how to fly without wings. Well, it isn't flying, you're just going ballistic. Good luck with landing safely. The universe doesn't bend around us.
Horsepower is made in only three ways: Make a bigger explosion, turn higher RPM, or make the engine more efficient.
1. Bigger Explosion: This can be done with higher compression, literally squeeze fuel mixture into a tighter space. This is how atom bombs can blow up whole cities. You can increase the amount of fuel mixture going into the engine with intake and exhaust: More air and gas sucked in means a bigger explosion. The more exhaust gases you push out means more room for air and fuel to get sucked in. You'll need a good rod for the compression. You'll need a good exhaust pipe as mention. Both of which you have eliminated given the parameters.
2. Turn Higher RPM: HP = (Torque * RPM) / 5252. Double the RPM, double the horsepower. Simple. Don't go over stock RPM with a stock rod and flywheel--things you have eliminated given the parameters outlined in your original post. You can adjust the governor with the throttle stop screw, but still you're limited.
3. Make the engine more efficient. Higher compression makes an engine more efficient. A lighter rod will make a more efficient engine because the engine will spend less energy keeping itself running. Again billet rods are great for increasing efficiency, but again that isn't a part of your parameters.
Your only option given these parameters would be to install a good intake, rejet the carburetor, and turn out the throttle/governor stop screw, keep the stock exhaust. That might get you an extra 0.5 hp maybe and 0.75 extra horsepower. You won't see much better performance over the bone stock engine.