I think the block will change over, but it will be sleeved and won't have that magic 'GP' cast into the block.
It's the peripherals that make the difference: you can't just take a 9N from a tractor, you have to transfer all the peripherals to make it into a GP engine.
The valve guides are available as part of an overhaul kit from David Tractor at a pretty reasonable price:
http://store.ford8n.net/product.php?xProd=6315
Mike