Nitrous is probably the cheapest but can be dangerous if done incorrectly.
Cam and heads are where the power is at. Like was said, get a small cam (duration wise) that will give you some nice low end torque. You can make some good power however you are somewhat limited as its a 305 . . . a fairly small displacement motor.
305 or 350 which is it? They are not the same and will make power according to what modifications you make to them. Go to the right front of the engine behind the altinator for the flat plate with the stamped numbers on it and write down the numbers and look them up and see what motor you have to begin with.
Acctually it was 305-350 not 305 and 350, that includes 305,307,327,350 i think he was refering to small block chevys in general. rebuild it with low compresion pistons and pop a blower, headers, hei distributor, rebuilt heads, new exhaust system, just take it somewhere hand them a huge wad of cash and ask them to make it fast!!!