what heads to use?

I have a 72 340 and the head that are on it have casting numbers of 1349 4027593 and I looked up the numbers and it came up with 318 heads?someone must have swaped out the origanl heads why??
and I have a 1973 360 with j heads on them.
the car im putting the 340 in is a 1970 duster that is a 340 car.