65 barracuda Commando 273 /Formula S?
You could probably get a running 360 low(er) mileage for $400 or less with the larger mains.
All I am saying is the 360 is a cheaper and easier motor to find. There are a lot more of them out there. If you can find a 340 for $400 it probably has already been rebuilt once or at least it has been beat to death or your a very lucky person and somebody doesn't know what they have. If the 340 is running and not beat to death you can probably get $800 for it as is.