I'm not sure about a 55 GMC but when my buddy down the street did his 49 chevy -3100 also with a 73 nova we put the innner fenders on bolted to the cab and did a mock up of the front end we spaced the rad suport off of that and than bolted the front fenders on,than we than could see if she was all lined up .I could give you the height of his rad support but depending on how and where yours is welded on the frame it could be different,make sure that your cab is set frist.
Pete
