I have an o1 HD2500 with the 6.0L in it. But am looking for a duramax. Should I buy the motor and allison used and put in it. Or just go buy a new truck all together. Im trying to do this the cheapest way of course.

I would buy one. Not only would you have to mess with the motor and tranny, but the computer as well.

Get a new truck with the duramax,here are just a few the duramax/allison combo wont fit unless you raise the body up of the frame about 2 inches,allison tranny is bigger then your tranny,also the computer and wiring harness will have to be changed,fuel pickup pump at the tank,intercoolers in front of the radator for the turbo will be needed,your rearend will prob.need to be changed yours is a 4:11 duramaxes come with a 3:73,4:11s will rev the d/a up pretty high at about 60-70 mph,gonna need a differant driveshaft and the list goes on.

Cheapest way is by far to buy a truck. Conversions always pencil cheaper, but if you do it right the first time, it always costs a LOT more to convert than to buy the same rig. Been there, done that - learned the hard way!!!

If you could find a truck that was rearended hard and can't be fixed, it MAY be feasable from a cost standpoint. I guess it depends on how much you love your '01...

You may need a rearend upgrade. I believe your 6.0 HD doesn't have the 11.5" rear. Not to say if you didn't beat on it it would have to be changed immediatley.