GM Duramax vans get the 6L90 transmission, or at least started to once they went away from the 4L80 which was I think around the time they went to the newer LML based LGH engine code. The downside of this is yes, that motor then got more emissions equipment added to it so you could look and see if there was a period of time where you could get the 6L90 and the older engine code without the DPF etc. I have around 135K on my van and often carry loads and do some decent towing at times and the 6l90 has been great. I've flushed it once I think...maybe twice but just for general maintenance but done nothing else to it. I do have a mild tune in my van as well and not always a very light foot...its gets worked and has been flawless.
The Duramax engine is, in my opinion, the best engine you can put in one of these depending of course on how much you're willing to pay for it. At least at this point anyway as the 7.3s are getting pretty old. Its very quiet, like as quiet as you can imagine a large displacement diesel being sat right next to your leg. In fact I'd go so far as to say its more quiet than the gas GM vans I've driven and ridden in. Fuel efficiency is solid and is likely as good as you're going to do in a full sized van particularly if you put bigger tires etc on it. I get around 15 running 70 mph with 33" tires and roof rack. Thats regardless of how much weight I have in it. Driving around town all day it never does worse that 13. You'd got to figure that with the cost of diesel (which is cheaper now at least here than gas) and more expensive oil changes. I'm obviously a fan....if you can square the cost its a fantastic engine.
Ride quality in the IFS is so much better than the solid front ends in all the other conversions I can't even begin to tell you. I daily drive mine and its as smooth as any full size vehicle getting around town and on freeways over joints etc. I've had no issues with mine failing or causing problems. I've had to change the ball joints and the CV axles but only at a rate I consider regular maintence. Yes, the front won't articulate as much as a solid front axle but I'd actually argue there is more ground clearance up front. Driving washboards and forest roads etc. is a much improved experience over a sold axle. Ok...there's my two cents which ended up being more like two dollars...sorry for rambling.
