You're correct, the overall height of the sending unit would increase by the thickness of the unit (plus gasket). In this case probably about 1/4".

Actually, if you wanted to do us a favour, you could measure the gap between the top of your sending unit and the underside of the access panel thing. Now that I'm thinking that might be tough to measure but I guess the underside of the sheet metal surrounding the hole is measurable. This info is useful for figuring out how much room there is for fittings attached to the sending unit, etc. Thanks in advance.