Major modifications? Well, you run pedistal mount rockers right?
351 heads will need to be cut for screw in studs and guideplates, then you will need a valve job depending on the mileage, mine needed new vavles as well, then a set of new rockers and pushrods. Plus the additional cost to mill them. I would say it's not worth it and you will gain very little, go with aftermarket heads, I wish I had the first time.
