will they work?

You should check with your friend on what heads are presently on the motor. You can easily sell the 302 heads and put money towards the 340. I have a 318 that I'm installing 302s on, along with some other work and parts, but make no mistake about it, if a good running 340 were available to me for a very good price I'd jump on it in a NYC minute. JMO, Mike