Here are my vies on the subject and the points defined.
Fuel economy: The 2H is a 4.0 Liter engine, and the 3B is a 3.4 Liter engine big difference, i personally don't consider a fuel economy a issue, because i try to take recreational vehicles in a different way, but thats me and i cannot judge anyone based on my own beliefs. I think a 3B would certainly qualify as a better, fuel efficient engine.
Power in Low Gear: That comes down to low end torque, and overall torque as well. 2H produces roughly 151 ft-lbs of torque at 2200 RPM while the 3B produces 170 ft-lbs at 2000 RPM, so definitely 2H is the performer here.
Power on Motorway: That comes down to output power and the maximum RPM the engine puts out. 3B puts out 90hp at 3500 RPMs while the 2H puts out 103hp at 3500 RPMs, so certainly the 2H would be best.
I personally think that a powerful engine would make lesser noise because it will be putting lesser of the effort, but the 2H being a 6 cylinder engine it would be noisier than a 4 cylinder. I think both would put out the same noise levels.
Availability of Parts:
Not sure about that because i haven't had the opportunity to own a 60 Series LC.
But i'm under the impression parts are not very hard to get especially for Toyota.
If i was in your shoes, with that much effort put in the LC, I'd certainly stick a 2H in it, because it certainly deserves it. After all its the Mighty LC 60! it deserves at least a 2H engine but that's only me
Overall, i think a 3B would satisfy your needs perfectly. With your usage described go for the 3B with your eyes closed.