This problem is very common on 98-99 F150s. The blokc is poorly machined and does not seal properly with cylinder head. So after a few miles the gasket starts to leak. If they change the gasket only, it will leak again. You need another engine. There are numerous threads at http://www.f150online.com Thet guys on that site will be more helpful than here. Most of them are dealing with the problem as we speak.
Personally, I would not rest until Ford changed my engine. That is the only solution to your problem. Replacing the gasket will work for a short term and then you will get another leak.
Try going to another dealer and they might be abel to help you.