4" exhaust is more than enough to support upwards of 500-600HP in a DURAMAX. The VAN TCM was meant for a 4L80E transmission behind the DURAMAX, but I don't think you can get a calibration for the T42 TCM to work with an LB7 engine. Also EFILIVE will most likely be a must for you since you are going with a NON-GM factory combo. The ECM will be looking for feedback from the TCM if it has a calibration in it for an ALLISON, and if it doesn't see it the engine WILL go into limp mode which will give you 0 boost and a 2000RPM limit. You will have to get a manual transmission calibration and then have to tweak the idle fuel tables so it doesn't run away on you when you drop it in gear. Since you are using an 01, you are pretty well limited to 01 for options. The van TCM was an option in 06, but GM changed alot of things in 06. I know you are in a hurry to get this done, but the ALLISON will simplify almost all of the custom tuning needs that you will incure if you run the 4L80E. What is so special about your current transfer case? It should be a BORG WARNER 13?? series I would think since it's mated up to a 4L80E. I would go with the ALLISON/261 combo and then go with a fixed yoke for the front driveshaft and be done with it. This will eliminate the need for custom tuning, and it would be a combo that the GM software will work with as stock.

I'm not trying to be a buzzkil, but I think you're setting yourself up for a major breakdown with your current combo. And it is not any fun at all to have to go back in and re-do part of your swap.

I tried to PM you his number, but it says that I can't. Type "INGLEWOOD TRANSMISSION" into most any search engine and it will bring you to his website.