you as autocityman4 has said, the compression would probably be 11- 12/ 1 compression and the rule of thumb max for running pump gas is about 10/1 compression, therefor, you would ither need to use just a little laquer thinner in the gas ( toulene and xylene, main ingrediants, big octane booster, ingrediants used in most off the shelf octane boosters ) but I wouldn't recomend that, think of it this way, the 305 heads you have, will raise the compression and lower the performance because of the port size diffrences, you could buy a new set of Vortech heads from any GM dealership, for around 600 a set, or if performance isn't a problem, any 350 head would work fine, some might limit performance like a stock 80's caprice or somthing set of heads but, a set of 461 heads would be nice.