i have a set of heads that i rebuilt for a 350 chevy. all machine work was completed to factory spec. I used hardened valve seats on the exhaust to handle the increase temperature of the propane. What makes these propane heads do you ask? well its this. I took heads off a 305, which have the exact same size valves as a 350. There will be no loss in air flow because of the same size valves. I magnafluxed the heads to check for cracks and there are none. In fact the only difference between 350 and 305 heads is the size of the combustion chamber. A three angle vavle job was completed. The 305 heads will increase the compression ratio, not sure how much but ive been told from 1.00:1 to 2.00:1 compression increase. This will help compensate for the power that comes with propane. I used all new parts when rebuilding these heads, except valves. I used perfect valve stem seals so the motor will not burn oil. My number is 916 521-8114 and im located in sacramento california. i am asking $300 obo. All of the work performed on these heads was done at Sierra Junior College with the help of a master machinist. If the buyer arranges it i will ship them. Feel free to call and ask any questions.