Just remember this is your story and you can tell it however you like. The six to eight conversion cost depends on who does it. You, or someone paid to do it. Buying a same condition v8 car for the price you get out of your original is a fantasy unless someone really wants it for pure 100pt resto. More likely the new owner will put a v8 in it so you're right back at the starting point. I suspect you are building the car for yourself, not eventual resale. It's your statement, your fun, your toothy grin showing through the windshield.