Actually, 305 sbc heads will bolt on a 350 sbc, however, even though both 305 and 350 sbc engines share the same stroke (3.48), the Cylinder bore is obviously smaller on the 305, therefore the Cylinder head chamber size was designed smaller. Yes, this will increase compression but it causes other problems that are actually determental to performance. Because of the smaller chamber size, the walls of the combustion chamber tends to "shroud" the valves and does not promote good intake and exhaust flow when installed on a larger bore engine like the 350. This also causes a problem with combustion flame travel. Do the right thing and get the right 350 heads for the application you want, there are a lot out there and they are cheap! After all, if you don't have the time to do it right the first time, when will you have the time and money to do it over! GOOD LUCK ON YOUR PROJECT.