if you don't care about the fuel injection you can get 300-350hp outta that ole 305 without too many problems. edelbrock performer air gap manifold, 650 cfm carb of your choice, headers a crane saturday night special cam, a good regulator so you can drop the fuel pressure to about 5psi, after market hei dist and a cowl hood so it all fits. you can do the whole swap in a weekend if you have a decent set of tools.
you could get a little more out of it with a set of world products or edelbrock aluminum heads.
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if you decide to swap for a 350 your going to need the tbi unit from a truck and the computer the 305 setup won't provide enough fuel. if I was goin to go to all the trouble to do a motor swap though I'd look for an LS/LT-1 its a little more work but you'd suprise some people good luck with your project.