Damn, wish I had seen this thread this morning, I literally just got back from the body shop to check out how the fitment of the HD hood on my spiral gray '03 is going.
This is what I can tell you.
1) I'm using an OEM HD hood off an '01 2500. My fenders are also factory original.
2) In order to get the fitment right, the body shop made a template of the headlight, then removed it, then bolted on the HD hood. Using the template as a guide, they made the initial cut, leaving about 1/2" extra for it to be rolled back (adds strength to the lip of the hood). Small amounts of body filler were used along the cut areas to smooth and blend the rolled portion of the lip to match the rest of the factory roll.
3) The fitment with the GMC grill is not perfect - the front edge of the hood sits slightly more inward than the stock GMC hood. There are no gaps, but just something to keep in mind.
4) With the HD hood mocked up for fitment, it doesn't appear that there will be any contact between the fender and the hood near the hinges, as some have people have brought up. However, this may truly be a truck-to-truck variance, as I've seen all manner of body gaps between the fenders and hoods on NBS trucks.
I will do my best to swing back by the shop tomorrow and snap a few pictures before the hood is fully primed and painted, and hopefully that'll be of some help.