Crumm had good advice...... BUT if it was ME, If yer already payin about 200 for the downpipe and delete stuff, yer only a hundred away from an entire new exhaust.
I would reccomend an entire new exhaust because if you ever wanna git one later, you will have wasted money, because you will have already bought the dp. Besides a new exhaust sounds and looks cool
You should be able to get a good Aluminized 4" Turbo back for $400. I have alot of experience w/ the diamond eye brand kits, and they are excellent quality, plus install is great. But be forwarned that any downpipe install on our trucks will make you do the infamous firewall "massage"
http://www.diamondeyeperformance.com/products.asp?cat=exhaust&subcat=ford
However.... Yer young like me. I'll bet you like the idea of cool sounding exhaust. 5" on our trucks sounds sweet.... Ive never done any business w/ these people, but there is an add in diesel power for a 5" exhaust system for only 369 and their website said free shipping on a bunch of stuff.
http://www.rpmoutlet.com/powered5.htm
Best of luck!!
-Stouff