Txchevy,
I agree with Techinspector mostly. He mentioned compression ratio and that is the biggest prob. Most 350 heads have have a 70+ cc chamber volume while 305 have about 55 cc chamber. I don't know if the motor will even crank with that much of drop. 305'd had about 8.25 to 1 or so c.r. and you would end up with around 6.0 to 1. The other problem he mentioned is valves. The 350 valves have a different angle to cylinder and even if they clear the top of the bore, they may hit cylinder wall. If the problem is $, you would have better luck haveing the 350 valves put in 305 heads (if they are not to big). Other wise World Products make an excellent 305 replacement head that with a cam, intake and headers wakes up the 305. Just my two cents worth.