I have been trusting the word of the guys that tell you not to cut the heads for over twenty years and they have NEVER steered me wrong. Just the opposite. If they say it can't be done, it is for a reason. I don't do my own machine work, but know you will have to cut MORE THAN .840 material off the head and intake combined to come close! Measure through an intake bolt hole on a 270 head and you will see there may not even be that much material to start with.
This reminds me of the cowboy that jumped into a cactus patch. When rescued he was asked what the heck did you do that for? He answered, well, it seemed like a good idea at the time!
Sometimes its' just better to trust others than our own ideas.