I'll have to agree with your builder. if you have .700 lift, theres No Way your going to be able to drive that thing on the street. I havent heard of anyone complaining about valve stretch due to cam lift, but i imagine it may lead to some valve issues. If it was me personally, i would consider .600 the limit. I would reccomend about .550 lift. Unless you plan for this car to be your absolute race-and-only-race car.