I think the best way is to pull the motor and turn it into a 383... but that might be out of your scope.
You may have to pull the motor to change the camshaft, depends on if there is enough room or if your back bench is removeable. Your not going to get too much out of just a cam change, but if you change the cam and have head work done (or new heads such as aftermarket or vortech) you will net gains.
An aftermarket exhaust system is the easiest way to get a few mph out of it.
If you do all of the above, you could have a reliable 383 putting out 400+ hp, and with more torque everywhere. Head and cam improvements alone will make a good difference. Depends on what you are willing to invest in both time and money.
The stock GM hi-rise intake is actually pretty good, the aftermarket stuff won't get you much, if any.