I installed a new top last summer. It went on easy, I had my wife help: Key was was good prep, clean surface, patience and doing it a 1/4 at a time. Low humidity is a plus
When I laid it on as a test fit, yes, moving it back and forth will alter that measurement. So a historical OEM measurement will def help, but Ill bet measurements from the factory originals will vary. Remember, allot of stuff was virtually slapped on as Q/A wasn't exactly rampant back in the day