Buddy, I've been around the block more than once w/ exhaust on these trucks... I've had no fewer than TWELVE different arrangements on my truck in the past year! LOL!
first, if you can "make the rest", you can make a downpipe no problem... there's tons of room to work, and you just need one bend... I don't know what the exact degree of bend it is, but you can cut it out of a 90* bend. and just weld a 3-4" adapter as close to the turbo as possible... 'course, these days, prices are probably low enough on a downpipe, it might make more sense to just buy one.
anyhow, one thing I've found that seems to REALLY cut down on resonance is to damn near hard mount the exhaust to the frame... use some really stiff hangers w/ rubber isolaters... and then use a good length of flex pipe...
I just went through trying out THREE different mufflers on my stack setup... each one yielded MORE interior resonance than the straight pipe... I've got 41" of flex pipe between the downpipe and the bed w/ the straight setup... with the mufflers, I could only squeeze ~10-15" in there.
but I'll tell you, the thing that made a HUGE difference was to add a bunch of sound deadener to the interior sheet metal... I would almost be willing to bet that your current setup would be fine if you did that.
anyhow, for your setup, I would go with a 51" donaldson, and if you want furthur reduction, a resonator of some sort right after the downpipe. I've got a 4" dynomax bullet I'll sell you cheap (like I said, I've been pulling my hair out trying different mufflers) and run a good length of flex pipe between the span.
FWIW, none of the systems out there use the 51" muffler, so you're probably better of pieceing together your own system.
but like I said, don't forget the sound deadener... at the VERY LEAST, add some to the back wall of the cab. there's NOTHING back there, and it just vibrates like a steel drum.
my truck with 5" straight pipe and lotsa sound deadener was quieter inside than it was when I had a 27" long muffler but no sound deadener!!!
Forrest