Allison Swap into Duramax Van

This is specifically a question about doing the swap. I don't have EFILive yet, but I suspect it will end up being involved. If a custom OS purchase is required, I'm ok with that.

I have a 2006 Chevy Express with Duramax and 4L80E. Yes, GM built it that way. I want to swap in an Allison... mostly for the bragging rights.
It has VIN code, which should be LLY, but it has a Bosch ECM. The transmission controller is a T42.

A few people in the van community have tried the swap, and not gotten it to work right. Getting detailed info about exactly what they tried and exactly what didn't work right is difficult. One swapper tried and said that the transmission would partially work with the van tune in the ECM, while the drivetrain would work correctly with the truck tune in the ECM, but Class 2 comms with the instrument cluster would not work correctly. I do not know the exact years and applications he was using.

Does anyone know how to go about this or what the pitfalls are?
I have read that "there are no 2006 Duramax ECM's that kept J1939", but a whole lotta people don't know there were Duramax vans. The shop manual shows two wires for the high speed bus, so it definitely has a high speed bus. I just today found out that the Vortec 8100 + Allison vehicles did NOT have a high speed bus, even for 2006... I think.

I know about the J1939 to High Speed GMLan change for 2006, as well as the differences between Class 2 and Low Speed GMLan.

So my van ECM *should* speak HS GMLan, relfash modules over high speed GMLan, but still communicate from the drivetrain computers to the body with Class 2 data.
Do the T42 and Allison use the same vocabulary? If I connect the *correct* Allison TCM to the Van ECU, will it just run? Do I need a Medium duty TCM instead of a pickup truck TCM?
I am aware that the van instrument cluster will not give me any feedback about what the transmission is doing. An adaptable tap-shift shifter handle exists, so I can implement tap-shift. The van also has a tow/haul mode, so that function is present as well.

These are a couple of the informative threads and posts I've read:

https://forum.efilive.com/showthread...l=1#post211055

@TheFermanator may not appreciate this one as much :thumb_yello:
https://www.duramaxdiesels.com/forum...plained.55510/

Thanks in advance!