It's definitely a complicated job. But can be done. Success or failure would also depend on your definition of success. If you just want it to run, and move under it's own power, and that's about all you care about, it's not too hard with a 300tdi. Having the vehicle run, and be reliable like it came that way from the factory is a whole other matter. To get a 300tdi installation to integrate with the factory computer systems would be difficult. For example, if you want to retain the ZF auto transmission, you'll probably need a Compushift from Ashcroft. Getting all the functions of the dashboard to work might be a practical impossibility. For example, the tach is driven from the ECU. The ECU will not work at all if you have a 300tdi installed. So, the tach will be innoperative. The speedometer is driven by signals from the SLABS system, and the SLABS system wants to see signals from the ECU. So if the ECU is not working properly... Coolant temp gauge also comes from the ECU.
Another proposition is to install a Td5 engine. This, I believe is more complicated to get running than a 300tdi, but easier to make it fully-functional (even to include cruise control) and offer factory reliability since the finished job should use all OEM parts, nothing cobbled.
The best way to go about this would be to acquire and entire D2 Td5 from Europe. Cheaply done if you purchase a wreck or MOT failure. I'm not sure of the logistics of shipping such a vehicle, but you could also pay to have it cut up and shipped in a crate. The list of parts is extensive, and you'd really be taking the entire front of the truck. Plus the fuel tank and fuel filter.
One complication to consider here is that most higher end trucks in Europe came equiped with the factory alarm system. A Td5 ECU with a factory alarm will want to see a BCU with an alarm. So, you need to swap over the BCU from the diesel truck as well. That also means you need the alarm sensors from the donor, such as the volumetric sensor which is located on one of the B pillars.
It may seem like a lot of work, but I think it actually ends up being easier than bodging in a 300tdi.
. The only problem is that the truck has a bad engine. 
