I don't know if it can take 350lbs of torque but the axles/diffs and the tranny can handle more than the stock motor puts out. The Aussies been putting big block v8's in their Patrols for years with stock drivetrain with little to no probs. Nissan diffs and tranny/transfers are known world wide for being extra strong.
The old C216 axles/diffs are a fairly low spline count. So that is one weak link. In the front diff though it uses the old tracta-joint. The tracta-joint is what you find inside your swivle hub or also known as knuckle flange. A tracta-joint is much stronger than the newer design which is cv joints. people break CV's all the time but I have never heard of any person breaking a tracta joint. In fact I would like to hear one person say they have broke on....
The only time I have heard of people breaking rear diffs/axles was due to operator error. You can do it in any open diff. Lift a wheel, power ends up going to the lifted wheel. Truck comes down on the spinning lifted wheel and bam...broken axle. That is pretty much what you have to do to break them.
Suspension sag from a heavy diesel motor? Easy fix...add more leaves to the front pack to compensate for the extra weight.