Gutting the cat in a earily car like that will not throw a check engine light. The ones that do have a set of O2 sensors er pipe, one is a effiecency montor, that tells the computer how well the cat is working.
Now on your car you have a few things not in your favor.
#1 its heavy
#2 the 305 is a destroked 350 for the most part
#3 its a chevy
Now the good, it is a old dog that can be trught a few new tricks. So cover the basics, more air in, more fuel in, use max engery from fuel and get it out faster. So I would say a free flow air intake system, some headers and a larger exhaust, or go all REO Speed wagon and get some straight pipes with mac daddy glass packs!!!
Now if you want to make some real power for low bucks transplant a LS motor in it with a hair dryer. 900hp and drive it on the street. I am a Ford man to the bone, but you just cant argue with what the LS guys are doing on the cheap!!!