To do it right, you need a forged crank because the cast ones are more prone to breaking due to the increased torque.
The 4.2 block is very similar to a 3.8 but wall thickness is more uniform, stronger ridges etc. You need to fabricate a bracket for crank sensor and reposition the dipstick. Engine mount, accessory brackets all fit. Knock sensor may not work.
The process to have it built, rebuilt and rebuilt took far too long and too expensive. Balancing the rotating assembly is critical and there are a few philosophies on how to do it (so I am told).
Tuning has been a PITA. And I am not impressed with the performance given all the MP bolt-ons, underdrive accessory pulleys, overdriving sc 15%, bigger injectors, lightning maf, 8.7 cr ...