I just finished mounting my FT-90R in my new RAM 3500 Diesel, and ran the power leads (both positive and negative) through the firewall to the battery on driver side of the engine compartment. It originally was mounted in a RAM 1500 (V6 Gasoline) using the same method. So even with a standard alternator that comes with a non-diesel, I do not think you would encounter any issues powering your radio using the mentioned method. In both installations, I have used the radio with the vehicle running, as well as turned off. Just remember to turn the radio off before you leave the vehicle
On another note, I just ordered the TS-480HX, which has a potential current draw of 40A. I plan on installing that into the truck as well. In doing so, I guess I will find out just how much the higher rated alternator will actually handle. I would be interested in finding out if anyone has had experience with this radio in a RAM 3500 Diesel?