With the demand being almost at panic levels for the last of the good 'ole 5.9s, I would suggest putting your truck on ebay or listing it for sale, as you may get a bit more for it. The same applies to the mustang. You would probably be in a better financial position to negotiate a better deal. Also, shop around and work the deal a bit.
Having said all that, I personally recomend you purchas a 6.7 with the 68. great combo .
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2007 3500 Dually 4x4 Laramie 2FH package,6.7L,68RFE, limited slip, 3.73, sunroof, kicker amp and 2x10", 2500 watt inverter,tint, back up camera,B&W G/N, Edge Juice W/attitude all over a MP-8....,BADP'S 5" TBE ,S&B Air,.....