Regardless of the age of the clock the internal movement will need to be secured against movement whilst in transit. The weights will need to be removed.
At 8ft high the item is too tall to be transported standing up either by air or sea - particularly as it will be considerably taller once crated - a wooden crate is advisable. Whilst there are specialist transport methods for over height items they are less frequently available and much more expensive than standard transport options. If the movement is removed the case can then lay down (horizontal) during transport without harm.
There are specialist packers available in the UK and some freight agents have specialist packing divisions - I have used a company called 'Allport Cargo Services' for the transport of all types of clocks before and their packing is very good. They offer sea or air transport. They can also arrange delivery and installation, including removal of the crate and other packing materials at destination.
My contact there is Chris Taylor (email firstname.lastname@example.org).
The age of the clock matters as it changes the documentation that is required for the import.
If it is an antique (over 100 years old) it should be zero rated for import duty. If it is under 50 years old duty will be payable. There is no import equivalent of VAT into New York.
The composition of the clock is important as some materials are controlled. If the face/dial of the clock or any part of the case is inlaid with precious woods, mahogany, stones or ivory CITES permits will be required. You need to know the species of the materials if at all possible.
For an antique you could obtain an estimate of value from a reputable auction house.
If you would like to discuss this further please do not hesitate to contact me.