look at the accesory end of the heads. there will be bolt holes that are 5/8 thread and some that are 7/16 thread.
They are also part of the EGR passage.
If you get the heads on bacwards, you will need to get the thread inserts to bring them down to 7/16thread.
Now if you have non EGR heads, it may not have the larger holes.
Other than that, the heads are symetrycal
Original Smog Pump Threaded Insert 5/8"-11 To 7/16"-14 | Cylinder Head Hardware | Engine | Mustang | MustangsUnlimited.com