Intrigue downpipe size?

does anyone know the downpipe size for the 3.5l engine? it doesnt really seem to be 2.5 or 3...and i have a 3inch hi-flo cat im wanting to put on, as well as materials to get rid of the u-bend.

any input would be appreciated, i know i could get a reducer go from the downpipe to the cat, but that would give me a grand total of 3 reducers in this little project, and im not sure how well it would all hold together.