if it was me i would not be to quick to pull the 305 heads, i mean i would not want 305 heads on my car but in a boat unless you race it why not try and use them. how is the timming chain ? i have know people to run 283 heads, on 350s with out any knock. but with the old heads exhaust seats may not last as long on the new gas. but you could ad in some lead or a little two stroke oil. if you're 327 is an old small crank its worth more