I have an older version of HP Tuners that allows me to work on any 1999 ECU. Question is, I have a 2005 GMC Sierra 2500HD with a 6.0L, would I be able to swap a 1999 equivilant ECU into my truck so that I can modifiy the settings for my truck aswell?

 

Nope, different processor.. only the 03+ will work in it. 99-02 has their own diff processor