The 305 heads are your only real choice for the 305, unless you put in flat top pistions, there 58 cc chambers and yield a 9.5 to 1 comp ratio on a stock 305, ising 64 cc heads would be acceptable but lower the compression to 8 to 1 ( approx ), 76 cc heads would be horrible. just do some head work to the 305 heads, it is the best option you have, other than getting some 49 cc heads to bump the comp ratio to somewhere around 10 to 1.