Good question
My shop has done a lot of these conversions and my experience has shown is that if you are not going to tow, and your Cummins power is not turned up too much you will probably be fine to run your 350 radiator. If it is not sufficient then upgrade to the Big Block 454 radiator it will be the same size as your 350 radiator just thicker. Only a small amount of trimming to your core support at the corners of your core support, and drilling some new holes on your existing upper radiator mounts is required to get the perfect amount of clearance on the Dodge Gen 2 Fan and Hub. I go over some of these details on our youtube channel jonesysautoclub
Since, your truck was a gas burner to start with the GM 6.5L diesel radiator is not a good option as it does not have a radiator cap and uses a pressurized surge tank mounted on the passengers firewall. It fills through the lower radiator hose so you would have to source all of those parts. It is also the same size as the big block radiator with different lower water neck size.
I would stay away from the Gen 2 Dodge radiator and intercooler as it requires a lot of cutting and fab work to get everything to fit correctly especially if you are keeping your AC condenser. It is a nice setup if it is installed correctly it just takes a ton of work to do it right.
Also, since you are not going to be towing and not turning up the power. I wouldn't bother with a intercooler yet. Get your conversion on the road and broken in. You can always add that later if you need more power.
Lastly your handle looks familiar maybe we have sold you some parts already? If not we sell a lot of the parts to make your swap easier at our website:
1988-1999 Chevrolet GMC Truck Archives - Adventure Vehicles NW