you'd want any dead weight under the spring where it doesn't move, otherwise the spring needs to control that weight for no good reason
machining spring seat pads for alternate springs is common. if there's not enough material around the remaining perimeter you can use a locator cup to center on the outside diameter of the valve guide to hold the base of the spacer/spring in place. addition material equal to the thickness of the locator cup will also need to be removed to achieve correct valve spring installed height/pressure
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