Why? Don't let any of that stuff scare you off. Machining valve guides for retainer clearance is very easy. Measuring for correct pushrod length is too. There's nothing about any of this that you cannot do, or pay a machine shop a very reasonable price to do if you are uncomfortable with it. There's nothing to it that should make you rethink your plans. It's not that big a deal, really. You've chosen a great grind. You should follow through with it. I think it will run fantastic.