Does anyone know if a 1999 4.6 will interchange with a 1997 4.2?

 

not sure but i think the computer/harness whatever they got are different. and depending on what tranny you got, a different bell housing. if its an auto maybe a whole new tranny in general.

 

Welcome aboard, I recommend posting your question under the engine forums, (related to those two engine 4.9 & 4.7; someone there prolly has done that swap and be more knowledgable than I could ever be.
Again Welcome aboard.