Since it's snowing outside and I'm not that motivated to go out and work on anything, I'll summarize what I have done and what still needs to be done to make the 2nd gen (1994-2002) cummins radiator and intercooler fit in the 1997 K3500 core support:
1. Cutting out sections of the core support for radiator and intercooler fitment - cutting holes on both sides of the large center opening for the intercooler inlet and outlet, trimming back sheet metal on the top of the core support for radiator clearance. I don't have exact dimensions yet, but I'll will get them and detailed pictures before reassembly. I chose to cut the mounting flanges off of the side of the radiator instead of cutting out more of the core support. The upper radiator brackets look like they will work with the cummins radiator also, with some minor relief cuts for the end tank crimp. I can either drill holes in the bottom of the core support for the existing radiator insulators, or cut them off and try to use the stock rubber radiator saddles.
2. Radiator hoses and caps - upper hose works, no issues. The lower hose has an ID of 2.125", the Cummins lower radiator port is 2.25". I'm going to try to force it to work with the stock lower hose. There is no barb for the heater core return, so I'm going to attempt to add one by drilling and tapping a hole in the plastic end tank for the barb fitting, then slobbering epoxy all over it and hope it seals and holds pressure. There is also no steam vent barn for the hose back to the accumulator tank, so I may cap it off at the tank and delete the hose. There will also now be two radiator caps in the system, I think I'm going to find a high pressure rated cap for the radiator and forget about it. The accumulator tank is still the high point in the system, so it's best to fill from there.
3. Transmission cooler - my truck used both the internal radiator cooler and an external cooler behind the grill. I'm going to remove all of that, and replace it with 6AN hose and a larger aftermarket cooler, similar to what is done with the oil cooler hose upgrade.
4. Intercooler pipes - I'm going to buy a universal 3" intercooler kit and fab up the pipes from turbo to CAC, and CAC to intake. The CAC inlet and outlet are 3.5" OD, so reducing couplers will be needed, probably 45 degree ones. I have both down or front exit compressor housings, and I have a stubby upper intake from Quadstar. The rest is filling in the gaps with 3" tube.
5. Marker lights - the marker lights in the grill will need to be trimmed to clear the CAC end tanks. I purchased a set of LED lights because the seem to be more shallow than the stock marker light housings.
I wish I could make more progress, but it's all weather permitting right now. Here in about 4 weeks, I will get 6 weeks away from work for the birth of my third child, so all this may not be wrapped up until then. I still have to finish the battery relocation also, but I have everything ready for that.