I want to drop in the 6.6 LLY but that could be overkill. the 4.5L LMK will get the torque I want, but I don't get the part about not having an intake manifold. Aren't those necessary? 
SBC clutch for Duramax (or similar Single Mass)
Duramax factory Motor Mounts. You'll probably need the Dmax frame mounts, too
A normal aftermarket exhaust should work.
Cooling
you can use the stock cooling system.
Gas radiator is 2 inches shorter then the diesel, so that means the gas has les capacity
Use the stock GM LB7or other piping
You'll need the Dmax core support and A-frame (trans cooler) support, too.
Electric fans are not needed unless it's for race applications.
Fuel System
You can reuse factory Gas fuel tank. Just drop it and clean it out really well.
Do not reuse the fuel system. Trans plant the entire fuel system over. The diesel fuel lines are larger and flow better.
Modify fuel tank filler neck for diesel pump filler nozzle
Electrical
You will need the DMax wiring harness
Your PCM or current PCM will need a security re-flash, so that the modules will all have same VIN to "talk" to one another
Upgraded battery and/or go to a dual battery set up
Misc
- 2500HD torsion bars
You will need the electronic (fly-by-wire) throttle (gas) pedal.
the hydro boost and associated plumbing will need to be added -- check if your ABS controller will work with it
Custom driveshaft -- check into using an Ex-cab driveshaft
Tcase may not be up to the task, if its the NV240 series -- may need the nv260 series
This information was pulled from another forum and modified by me. not to say I couldn't answer your question. I just felt that this was more appropriate.
I hope this gives you a sense of what you're doing or getting into.
This guy does exactly what your doing. Mash Motors Inc.com they have done Avalanches.
Your suspension and breaks might be underrated for the extra weight.
Duramax factory Motor Mounts. You'll probably need the Dmax frame mounts, too
A normal aftermarket exhaust should work.
Cooling
you can use the stock cooling system.
Gas radiator is 2 inches shorter then the diesel, so that means the gas has les capacity
Use the stock GM LB7or other piping
You'll need the Dmax core support and A-frame (trans cooler) support, too.
Electric fans are not needed unless it's for race applications.
Fuel System
You can reuse factory Gas fuel tank. Just drop it and clean it out really well.
Do not reuse the fuel system. Trans plant the entire fuel system over. The diesel fuel lines are larger and flow better.
Modify fuel tank filler neck for diesel pump filler nozzle
Electrical
You will need the DMax wiring harness
Your PCM or current PCM will need a security re-flash, so that the modules will all have same VIN to "talk" to one another
Upgraded battery and/or go to a dual battery set up
Misc
- 2500HD torsion bars
You will need the electronic (fly-by-wire) throttle (gas) pedal.
the hydro boost and associated plumbing will need to be added -- check if your ABS controller will work with it
Custom driveshaft -- check into using an Ex-cab driveshaft
Tcase may not be up to the task, if its the NV240 series -- may need the nv260 series
This information was pulled from another forum and modified by me. not to say I couldn't answer your question. I just felt that this was more appropriate.
I hope this gives you a sense of what you're doing or getting into.
This guy does exactly what your doing. Mash Motors Inc.com they have done Avalanches.
Your suspension and breaks might be underrated for the extra weight.
