As for the 6.2........well lets me start first with I have been die hard cummins guy for 20 years now, and most everyone I talk to bad mouth's the 6.2, having said that I started doing my own research about 6 months ago becuase I haven't been able to get any straight answers. After countless hours reading books studying gm tech manual's and and reading information on the internet (more tec manuals), I have come to the understanding that these engine's where in fact designed to be work horses. the early years of the 5.7 diesel are the starting point of the gm failures with diesel's and by the time the 6.2 came out the reputation had been set. Then in 1995 with the introduction of the electronic injection pump and all its short commings, it set the stage for the reputation of all the 6.2's and 6.5's. After reading several articles on a turbo 6.2 diesel, these engines where beating the early dodge cummins in Horsepower and torque, beating it in the 1/4 mile and the hill climb with a 8000 lb. trailer, they where only 2 seconds behind the power stroke in the hill climb but 6 seconds faster than the dodge(early year of dodge).
From what I have read these engines are very reliable and can handle up to 20 psi of boost (with arp head studs), and 400 HP. the only two downfalls are bad harmonic balancers, this should be the first thing you check and repair if the rubber ring looks dry rotted or is squishing out, switching to a fluid harmonic balancer is strongly recommended. This is the only reason the cranks break is due to bad harmonic balancers NOT pulling heavy loads or increasing power. Also the cooling passages on the back of the block causes poor cooling when MORE power is added or HEAVY loads are towed, but there are higher GPM water pumps to solve this problem.
The marine 6.2's where pushing over 600 HP, they where the same block, crank, pistons, heads, etc. as the civi 6.2. The only reason it could handle more power is the marine engine has the whole lake to cool the motor, not a small radiator.
Kennedy diesel is a good place to start if your looking for more power.