For me it's the cheapest 87 octane, or occasionally some old crap I drain out of something, haven't feared that, either. A 305 has an advertised 7.75-1 comp ratio, octane rating doesn't matter a lick, run anything that says gasoline.
Some of my crap:
1963 C-30 flatbed dually 292/SM420
1965 C-10 LWB fleet 250/column shift 3 speed
1965 C-30 9' stepside 250/SM420/4.10 gears (my DD)
Website I made for my crap:
http://www.66submarine.com 
